Revealing Growth Schemes of Nanoparticles in Atomic Resolution

Communication pubs.acs.org/crystal

Revealing Growth Schemes of Nanoparticles in Atomic Resolution: Mapping Stacking Fault Formation and Distribution Shai Mangel,† Lothar Houben,‡ and Maya Bar-Sadan*,† †

Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel Peter Grünberg Institut 5 and Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

‡

S Supporting Information *

ABSTRACT: Controlling the growth process of inorganic nanoparticles, especially the kinetically driven ones, is crucial for designing tailor-made nanoparticles for various applications. Specifically, controlling the formation of stacking faults in semiconductor quantum dots is necessary, since stacking faults were associated with inferior optical performance. Ensemble techniques, such as XRD powder diffraction and optical absorption, can be insensitive to the formation of stacking faults and in certain cases might produce misleading information. Using as a model the thoroughly studied CdSe system, we exploited the well-known unidirectional growth of the Wurtzite phase in order to follow the structural evolution of two different batches of CdSe nanoparticles. We were able to get insight on the crystal growth stages, step by step, employing high resolution electron microscopy and focal series reconstruction. The different kinetics of the two variants were monitored using a statistical approach. The same approach can be used to provide atomic-scale information for any system exhibiting unidirectional growth.

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readily, breaking the symmetry of the particles and potentially altering their chemical reactivities and physical properties.5 The main challenge in tackling the atomic scale growth patterns of nanoparticles is the collection of large experimental data sets and their statistical analysis, while providing the characteristics of each nanoparticle at the atomic scale. Today, indirect methods are still the most common procedures to investigate the morphology and properties of nanoparticles. The averaging of ensemble properties hinders gaining insight into the basic growth schemes of colloidal synthesis. More importantly, the analyses of data from ensemble indirect methods are less suitable for nanostructures, where there are crystallographic phase transitions every few atomic planes.6,7 Atomic-resolution microscopy mainly provided information for a few particles per sample,2,8−10 thus limiting the discussion to only a few particular cases, due to radiation damage and a general lack of stability under the electron beam of the transmission electron microscope (TEM). Here we demonstrate that by using high resolution electron microscopy and focal series reconstruction, it is possible to get quantitative information regarding the atomic structure of data sets of CdSe particles. The knowledge gained provided new insights into the growth processes, which have so far been

ltimate control over colloidal nanoparticle growth, and specifically the control of stacking fault formation within them, is a significant step toward obtaining tailor-made nanoparticles at the atomic level. Although the colloidal synthesis of nanostructures has been researched for over two decades, this task is still out of reach. Currently, the main obstacle is the lack of direct characterization of the particles’ growth process at the atomic scale that will provide direct correlation between the synthesis and the atomic structure. CdSe is one of the most intensively investigated systems in nanoscience, due to its size-dependent optical and electronic properties, which make it an excellent candidate for various applications such as photocatalysis,1 biolabeling,2 and electronic devices.3 Furthermore, the ability to control with high accuracy the morphology of the nanocrystals and their crystallographic phase has made it possible to produce designed nanoparticles for various applications. To date, optimizing the chemical and optical stability of the particles is still a long-standing problem,4 attributed to the lack of control over the atomic structure of the particles. The control over the uniformity of the crystallographic phase and the stacking faults (SF) distribution as well as their elimination is a major concern in the field. In the CdSe system, it is usually desired to maintain the hexagonal Wurtzite phase throughout the particle, which is polar and thus easier to grow in a controlled unidirectional fashion.2 The Zinc Blende phase is cubic and isotropic, thus promoting isotropic growth. The two phases are close in energy and transitions occur © 2015 American Chemical Society

Received: April 19, 2015 Revised: May 19, 2015 Published: May 20, 2015 3114

DOI: 10.1021/acs.cgd.5b00545 Cryst. Growth Des. 2015, 15, 3114−3118

Crystal Growth & Design

Communication

Table 1. Morphology and Optical Properties of Batch I and Batch II CdSe Samples Size, nm (absorbance)

a

Batch I

3.48

Batch II

3.56

PL peak, nm

Length, nm

Average size, nm

Lattice parameters (XRD) nm

Deviation from bulk valuesa

FWHM nm

Width, nm

Aspect ratio

a

a

(TEM)

(TEM)

c

c

3.76 2.81 4.90 3.26

3.29 1.35 4.08 1.50

0.42599 0.70209 0.42696 0.70481

−0.87% 0.21% −0.64% 0.59%

580 22 580 24

Bulk CdSe values from ICSD card 620423 (a = 0.42972 nm, c = 0.70065 nm).

absent from the scientific discussions because of the limitations of the ensemble techniques. The new findings show that minor changes in the growth schemes, which may look insignificant, reproducibly provide specific dislocation patterns within the particle. By relying on existent knowledge of the unidirectional growth of hexagonal semiconductors, it is possible to follow the formation of SF over time. The new knowledge can be utilized to better model the growth schemes of nanostructures. The kernel of the present research is the linear relation between the atom column intensities and their atomic number, under specific microscope and acquisition conditions. Working at those conditions allows resolving of the individual Cd and Se columns. In addition, using reconstruction of the exit-plane wavefunction and the retrieval of the phase of the electrons, the signal-to-noise of the images is enhanced. The graphene grid background pattern was subtracted using Fourier filtering so that the location of the heavier Cd columns was identified due to their higher intensity over the lighter Se anion columns (see the Supporting Information for detailed description of the image processing). Two different synthetic batches of CdSe quantum dots were prepared according to the procedure described by Amirav et al.,11 with minor variations. In short, CdO, TOPO, and ODPA were loaded into a flask and heated to 360 °C, under inert conditions. The system was then injected with a cold (room temperature) solution of Se dissolved in TOP, initiating the particle growth phase. The main variant between the two synthetic schemes is that the Batch II underwent an additional stage of pumping of the initial CdO, TOPO, and ODPA mixture for 1 h, so that H2O residues were removed prior to heating and dissolving the CdO. In addition, Batch II had a slightly longer growth phase compared to Batch I (50 s compared with 30 s, respectively). The samples were allowed to grow until similar absorbance spectra were obtained, where the estimated size was similar (3.56 nm for Batch I and 3.48 nm for Batch II; Table 1). The average size measured by TEM analysis deviated strongly from the calculated values based on the optical absorbance since the main assumption of the calculation model is a perfect spherical shape of the particles,12 which cannot be the case for the anisotropic growth of CdSe particles. Indeed, the TEM analysis confirmed that the CdSe nanoparticles were elongated (Figure 1). Batch II showed more elongated particles than Batch I, with an aspect ratio of 1.50 and average diameter of 4.09 nm for Batch II and an aspect ratio of 1.35 and average diameter of 3.29 nm for Batch I (see Table 1). In the size regime of less than 4 nm, the optical absorbances of Wurtzite and Zinc-Blende phases are rather similar concerning the correlation of the first absorbance peak with particle size.13,14 In theory, and in some experimental work, the gap between the first (1S3/2 1Se) and second (2S3/2 1Se) energy transitions can be used to identify the crystalline phase of the

Figure 1. Reconstructed phase images of CdSe Batch I particles. The images were Fourier filtered to subtract the graphene lattice. Stacking faults are marked in dashed orange lines.

particles, where a large gap indicates the presence of the ZincBlende, while a more narrow gap points to a Wurtzite.13,15 However, there is no theoretical treatment to a mixed state which includes both phases. Accordingly, in our measurements, the absorbance spectra of Batch II and Batch I (Figure 2a) are virtually identical and one cannot derive information regarding the particles’ crystallographic phases from it. The photoluminescence spectra of the two samples (Figure 2b) are also almost identical, with the excitation peak located in both cases at 580 nm, and the full width at half maximum (FWHM) width is 22 nm for Batch I and 24 nm for Batch I (see Table 1), which indicates a narrow size distribution in both the samples. XRD patterns of Batch I and Batch II samples showed hexagonal symmetry (Figure 3), corresponding to the Wurtzite phase of CdSe. Missing information on individual particles, the seminal paper by Murray et al.16 from 1993 used the Debye scattering model and postulated what is today the prominent method for the characterization of CdSe nanoparticles by powder XRD diffraction. According to the suggested model, the number of stacking faults within nanoparticles is estimated from the XRD patterns by using the intensity ratio of the (103) peak of the Wurtzite structure relative to its two adjacent (110) and (112) peaks.17 In pure Wurtzite CdSe, the intensity of the (103) peak is higher than that of the (112) peak and similar to that of the (110) peak. In their paper, Murray et al. stated that according to simulations, the (103) peak is extremely sensitive 3115



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Figure 2. (a) Absorbance spectra of CdSe Batch I (in red) and CdSe Batch II (in blue). (b) Photoluminescence (PL) spectra (excitation wavelength = 440 nm) of the same particles described in (a) colored accordingly.

Figure 3. XRD spectra of CdSe Batch I (in red) and CdSe Batch II (in blue). The spectra were normalized to the overall signal.

to stacking faults in the structure so it is attenuated even when a single fault is present and it is affected also by the location of the defect. Their main conclusion was that their particles had a single stacking fault in the Wurtzite structure, located around the middle of the particle. The attenuation of the (103) peak became a basic characterization tool of the occurrence of stacking faults in the structure. Another signature of the existence of stacking faults is the attenuation of the (102) peak. In the present study, Batch I exhibited a pronounced (103) peak with no observed attenuation relative to the (110) and (112) peaks. In contrast, Batch II showed an attenuated (103) peak relative to its two neighbors peaks, the (110) and (112) peaks, predicting many more stacking faults in Batch II compared with Batch I. Furthermore, the (102) peak of Batch I showed a more moderate attenuation than the comparable peak in Batch II spectrum, predicting many more stacking faults in the Batch II sample. Several dozen particles of each of the CdSe samples were reconstructed in order to assign the polarity of the particles, which enables us to provide quantitative statistical analysis (see

some typical examples in Figure 1). After the reconstruction of the exit-plane wave function and the retrieval of the phase, the locations of the heavier Cd columns were identified due to their higher intensity over the lighter Se columns. The particles were divided into quarters along the hexagonal c axis, which is the prominent growth direction of the particles, the first one at the Cd-terminated side and the fourth quarter at the Se-terminated end (see inset of Figure 4). The collected quantitative statistical data allowed the construction of histograms that describe the occurrence of stacking faults within these four quarters. The histograms for the occurrence of stacking faults are presented in Figure 4, Batch I in red and Batch II in blue. The histogram analysis shows that while Batch I and Batch II samples have a similar number of stacking faults (0.80 for Batch I and 0.88 for Batch II), the distribution of the stacking faults along the particle growth direction is quite different. Moreover, both distribution patterns differ significantly from the model presented by Murray et al.16 which predicted the formation of one stacking fault located in the middle of the particle. 3116



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Figure 4. Histogram describing the distribution of stacking faults of CdSe particles in the 001 direction according to quarters. The first quarter initiates with a Cd layer, and the fourth one ends with a Se layer, which is the growth front (see inset for schematic representation). Red columns for Batch I (in total 35 particles) and blue for Batch II (in total 24 particles).

The fault distribution follows the well-accepted growth kinetics. The c direction is the growth front of the reaction. The faster growth occurs on the Se-terminated facet, which has higher surface energy, as it is the facet to which the ligands are more weakly attached, allowing access to the building blocks from the solution. Moreover, the 001 facet (the Cd-terminated side) contains Cd atoms with one dangling bond, while the last Cd atoms’ plane on the 001̅ (the Se-terminated edge) have three dangling bonds.15 Growth also proceeds at the Cdterminated edge, but at a slower pace, which can be only 1.5 times slower than the Se-terminated growth or even much slower, depending on the specific reaction.18 Since the growth advances mainly in one direction, i.e., from the Cd edge to the Se edge, we can follow the formation of the stacking faults through time. In both syntheses, the growth phase occurs immediately after the injection of the Se precursor, as the temperature rises from 330 to 360 °C, which is more suitable for the growth of the Wurtzite phase (usually considered above 350 °C). Therefore, the reduction in fault density observed as the reaction progresses is attributed to the gradual change in temperature. In both batches, the higher defect concentration at the Cdterminated side may be associated with specific kinetics which produces more faults at the Cd growth front18 or with the nucleus of the nanoparticle, which may have neither Wurtzite symmetry nor Zinc-Blende15 or even a complete Zinc-Blende phase.19 This initial disorder of the particle’s nucleus may be retained when growth takes place. The distribution of stacking faults within the particles is different for the Batch I and Batch II samples, demonstrating different growth processes. While Batch I showed significantly fewer faults in the particles’ middle section and then an increase in the fault density at the Seterminated edge, Batch II showed a steady decrease in the fault density from the second quarter through the third quarter and to the Se-terminated edge. Comparing the synthetic procedure

for both samples, it seems there are only minor changes: the loading of the CdO, TOPO, and ODPA is followed by 60 min of pumping under N2 only for the Batch II sample and the growth stage is slightly longer, 50 s instead of 30 s for Batch I. We offer a possible explanation to the different stacking faults distribution: once the heating for a short time mantle is removed, the growth still continues at a lower temperature, and the last atomic planes are deposited with greater disorder, leaving in both sample defects in the last quarter. Due to the highly kinetic nature of the reactions, the 20 s difference between the two growth stages is revealed to be crucial. The additional time before the reaction ended led to depletion of monomers and reduction in the reaction rate, allowing better ordering of the last atomic planes in the case of Batch II, in contrast to the Batch I sample. In summary, we have shown that by using aberrationcorrected electron microscopy and focal series reconstruction, it is possible to follow the growth process and unravel the SF distribution. Using the stacking faults analysis, we were able to monitor the addition of the atomic plans during the reaction more accurately. Furthermore, we were also able to correlate between specific synthesis conditions and the corresponding final products. By using a direct characterization method, we revealed major variations in the particle size and crystalline structure, which were otherwise hidden by the use of indirect characterization methods. These findings demonstrate the importance of this methodology to follow the growth process of colloidal nanocrystals and to correlate between their specific syntheses to the obtained products, and can be applied to other kinetic driven hybrid systems.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental section including image processing procedures. The Supporting Information is available free of 3117



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(13) Harrell, S. M.; McBride, J. R.; Rosenthal, S. J. Synthesis of Ultrasmall and Magic-Sized CdSe Nanocrystals. Chem. Mater. 2013, 25, 1199−1210. (14) Karel Č apek, R.; Moreels, I.; Lambert, K.; De Muynck, D.; Zhao, Q.; Van Tomme, A.; Vanhaecke, F.; Hens, Z. Optical Properties of Zincblende Cadmium Selenide Quantum Dots. J. Phys. Chem. C 2010, 114, 6371−6376. (15) Hughes, S. M.; Alivisatos, A. P. Anisotropic Formation and Distribution of Stacking Faults in II−VI Semiconductor Nanorods. Nano Lett. 2012, 13, 106−110. (16) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706−8715. (17) Guinier, A. X-ray Diffraction in Crystals, Imperfect Crystals, and Amorphous Bodies; W. H. Freeman, 1963; p 378. (18) Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H.-S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G. Compact high-quality CdSe−CdS core−shell nanocrystals with narrow emission linewidths and suppressed blinking. Nat. Mater. 2013, 12, 445−451. (19) Yin, Y.; Alivisatos, A. P. Colloidal nanocrystal synthesis and the organic-inorganic interface. Nature 2005, 437, 664−670.

charge on the ACS Publications website at DOI: 10.1021/ acs.cgd.5b00545.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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 This research project was supported by The Israeli Centers of Research Excellence (I-CORE) program, (Center No. 152/11), ISF grant 475/12, the Adelis Fund, and the European Union Seventh Framework Programme, under Grant Agreement 312483-ESTEEM2 (Integrated Infrastructure Initiative-I3). M.B.S. appreciates support from Dr. Dmitri Mogilyanski of the Ilse Katz Institute for Nanoscale Science Technology (BGU).

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Revealing Growth Schemes of Nanoparticles in Atomic Resolution

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