Shell Heterostructured

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Recent Advances in Colloidal IV-VI Core/Shell Heterostructured Nanocrystals Youngjin Jang, Arthur Shapiro, Faris Horani, Azhar Abu-Hariri, and Efrat Lifshitz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00994 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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The Journal of Physical Chemistry

Recent Advances in Colloidal IV-VI Core/Shell Heterostructured Nanocrystals Youngjin Jang,† Arthur Shapiro,† Faris Horani, Azhar Abu-Hariri and Efrat Lifshitz* Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Nancy and Stephen Grand Technion Energy Program, Technion–Israel Institute of Technology, Haifa 3200003, Israel

ACS Paragon Plus Environment

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ABSTRACT. Colloidal nanocrystals (NCs) have been at the forefront of scientific research and technological applications since their emergence in the 1980s. The limitations of single component NCs and the growing demand for wide electronic tunability switched the attention to core/shell nano-heterostructures (NHs). The NHs consist of at least two semiconductor compounds, exhibiting improved surface passivation of the cores and surplus electronic tunability beyond that gained by size confinement. Several synthetic approaches have been developed through the years for the formation of core/shell structures. This article discusses two recent synthetic approaches, cation exchange and the Kirkendall effect, for the preparation of core/shell NHs, focusing on IV-VI semiconductor compounds. Both cation exchange and the Kirkendall effect are post-synthetic routes which offer an indirect way to synthesize complex core/shell NHs, that are difficult to synthesize by other methods. Overall, the produced core/shell NCs show enhanced optical properties and improved chemical sustainability with respect to the corresponding pure cores.


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1. Introduction To date, nanomaterials have attracted significant attention in science and engineering fields because of their intriguing properties compared to bulk materials. The electronic energy levels in semiconductor nanocrystals (NCs) with dimensions below the exciton Bohr radius become discrete due to the quantization effect. This phenomenon leads to substantial variation of optical and electronic properties on the size of NCs, unlike the bulk materials which possess a continuous energy band and a fixed bandgap. The size-dependent optical properties (absorption and emission) of semiconductor NCs over a wide spectral region have been the focus of extensive scientific and technological interest during the past few decades.1-5 Among several preparation approaches, colloidal synthesis has drawn considerable attention because of its benefits, including relatively cheap production cost, simple set-up, mild synthetic conditions (e.g., pressure and temperature), good reproducibility and high controllability of size, shape and composition. Colloidal NCs contain an inorganic core and an organic exterior layer of capping agents to avoid particle aggregation and undesirable reactions (e.g., oxidation) and also to provide precise growth control. Surfactant molecules (e.g., carboxylic acids and amines) stabilize the surface by the coordination, leading to strong bonding, reduced surface energy and passivation of the surface traps. More detailed descriptions can be found in previous articles.6-8 Nevertheless, in NCs there are unpassivated surface facets,8 supposedly owing to the steric hindrance between surfactant molecules.9 The partial ligand coverage results in the generation of trap states acting as non-radiative channels, consequently decreasing the emission quantum yield (QY). In addition, surface oxidation can occur in ambient conditions, also leading to deterioration of the optical activities. Therefore, achieving improved surface passivation of semiconductor NCs is of key importance.


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One of the prominent strategies for tackling surface coverage is the formation of core/shell nano-heterostructures (NHs) by covering a semiconductor core with a shell of another semiconductor, and retaining organic ligands at the exterior surfaces. The protection by an inorganic shell endows core/shell NHs with chemical and spectral stability (e.g., increase of QY10-17 and suppression of blinking behavior18-21) relative to core NCs which are covered with only the organic surfactant. In addition, the optical and electronic properties of core/shell NHs can be tuned by adjusting the band offset between core and shell materials, offering a larger spectral window compared to that of the corresponding cores. Consequently, core/shell NHs are considered promising materials, providing additional features that are not readily achieved by pure core NCs. Numerous review articles22-31 in recent years discussed the synthesis and characterization of a large variety of core/shell NHs. Synthetic efforts for obtaining high quality core/shell NHs have been developed in early years, including co-precipitation,10, 32-37 successive ionic layer adsorption and reaction (SILAR),14, 38-39 microwave radiation40-41 and microemulsion42-43 (see scheme in Figure 1). Those early age methods are summarized in a few wide perspective reviews.23,

26, 28, 31

Recently, new post-

synthetic routes such as cation exchange and the Kirkendall effect were proposed, enabling the production of unexplored core/shell materials.44-47 The current document will mainly focus on the post-synthetic procedures and will be compared with co-precipitation process. This article aims to discuss the recent efforts for preparation of IV-VI core/shell NHs and the related formation mechanism. In Section 2, the article briefly introduces the conventional route, co-precipitation and a related representative example of IV-VI semiconductors (e.g., PbSe/PbS core/shell NC). The next two sections present discussions based on a cation exchange process


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(Section 3) and Kirkendall effect (Section 4). Finally, prospects for the progress and benefits of core/shell NHs are presented in Section 5.

Figure 1. Schematic illustration showing various synthetic routes to prepare core/shell NHs (M and E denote metal and chalcogenide, respectively).

2. Co-precipitation The colloidal core/shell NHs have been typically obtained by epitaxial growth of a secondary semiconductor shell onto an inorganic core.10, 13, 23, 36, 48-52 In 1996, Hines and Guyot-Sionnest reported the pioneering one-pot protocol for the synthesis of CdSe/ZnS NCs.33 After the preparation of CdSe NCs, a mixture containing Zn and S stock solution was consecutively injected into the same vessel containing as-synthesized CdSe NCs for in situ epitaxial growth of ZnS shell.33 This one-step approach reduces total reaction time, but the presence of unreacted precursors or by-products after the core formation can affect the following step of shell growth. Therefore, the co-precipitation route is usually conducted via a two-step procedure, involving a step of preparing core NCs, followed by a purification process to remove unreacted precursors or by-products, and finally a consecutive reaction for the shell growth in a different vessel.


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Subsequent to the work of the Bawendi32 and the Alivisatos10 groups in the 1990s, plenty of results for II-VI core/shell NCs via the two-step synthesis have been reported.49, 53-59 The core/shell NHs of the IV-VI semiconductors, such as PbSe/PbS or PbSe/PbSexS1-x are of significant importance due to their optical tunability in the near- and short-wave infrared spectral regimes.36,

60-63

PbSe/PbS core/shell NHs were readily prepared by applying the two-step

approach.62 Representative transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of PbSe/PbS core/shell NCs are displayed in Figure 2A, showing PbSe cores with a size of 3.9 ± 0.3 nm and the corresponding PbSe/PbS core/shell NCs with an average size of 5.3 ± 0.3 nm. The HRTEM image indicates high crystallinity and no distinct boundaries at the core/shell interface due to a close crystallographic matching between PbSe and PbS (with lattice constants of 6.12 and 5.94 Å for bulk PbSe and PbS, respectively).36, 61, 64 Typical absorption (dashed curves) and photoluminescence (PL, bold curves) spectra of the PbSe core and the PbSe/PbS core/shell NCs are presented in Figure 2B, showing the shift of the first exciton absorption and emission peaks toward a lower energy with the increase of the shell thickness. The PbSe/PbS core/shell NCs produced high-quality optical properties, such as a decrease in the full-width at half-maximum (FWHM) value for the absorption and emission peaks, compared to the corresponding PbSe core NCs, as well as a small Stokes shift. Figure 2C displays powder Xray diffraction (XRD) spectra of the PbSe core (red line) and the PbSe/PbS core/shell NCs (blue line), confirming that all peaks correspond to the rock-salt crystal structure (Purple and green vertical lines in Figure 2C are peak positions for bulk PbS and PbSe, respectively) and the peaks of the core/shell NCs shift towards the bulk PbS line.62-63


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Figure 2. (A) Representative TEM (left and right) and HRTEM (middle) images of 3.9 nm PbSe cores and 5.3 nm PbSe/PbS core/shell NCs, showing high uniformity and crystallinity. (B) Absorption (dashed lines) and PL (bold lines) spectra of PbSe/PbS core/shell NCs with a variable number of shells prepared by adjusting the amount of S precursor. The labels refer to the overall diameter (nm) and to the number of shell monolayers (MLs). (C) XRD patterns of the PbSe core (red line) and PbSe/PbS core/shell NCs (blue line). Bulk PbSe (green vertical lines) and PbS (purple vertical lines) peak positions are given at the bottom. Reproduced from ref. 62. Copyright 2016 American Chemical Society.

For achieving a high quality NHs, careful execution of experimental conditions was essential in order to avoid unwanted events such as nucleation of the secondary semiconductor and ripening of the cores. A previous study61 discussed the Ostwald ripening process when PbSe NCs were heated to above 70 oC, by tracking the position of the first exciton absorption peak. Therefore, in most cases a low temperature was implemented for a growth of a shell, substantially lower than the temperature used for the formation of same constituents as core moieties. Moreover, a controlled slow injection (e.g., syringe pump) of the shell precursors is


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usually adopted to produce uniform core/shell NCs. The Lifshitz group demonstrated PbS shell formation on the PbSe NCs by slow injection of highly reactive sulfur precursors (bis(trimethylsilyl) sulfide; TMS2S) in the mixture containing Pb oleate precursor and PbSe core NCs.62 Manual injection could lead to an asymmetrical shape of the first excitonic band, suggesting non-uniform PbS growth on the surface of PbSe NCs, which was later exchanged with an automated syringe pump injection. Furthermore, the type of Pb precursor and its concentration also affect uniformity of the core/shell NCs.62 The PbO compound was found to be an optimal precursor for preparing PbSe/PbS core/shell NCs with a symmetric absorption peak, compared with Pb acetate that resulted in an asymmetric excitonic peak with a long tail at longer wavelengths.62 In addition, a high concentration of the shell precursors led to a secondary nucleation, showing two emission peaks.

3. Cation exchange process Since pioneering work65-66 in the 1990s and the milestone achievement67 in 2004, cation exchange has been widely applied in the synthesis of NCs. This process involves replacing cations in the NC mother solution with other cations when thermodynamic and kinetic conditions are satisfied, resulting in the preservation of the NC size. It is noted that anions in the crystal structure remain unaltered during the process in many cases, due to larger ionic radius of anions compared to that of cations. To obtain core/shell NCs, the cation exchange reaction should be halted before full replacement occurs. Cation exchange reaction sometimes proceeds rapidly, so regulation of the reaction rate is essential for controlling the shell thickness. In contrast to the coprecipitation process, the nucleation event of shell materials seldom occurs during this process, but a structural reorganization might occur.68-69 Detailed elucidation of the cation exchange is


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found in other review articles.70-74 The present article represents two examples, PbSe/CdSe and SnTe/CdTe core/shell NHs which were recently published.44, 75-76 PbSe is very sensitive to air exposure, resulting in deterioration of the optical activities. Because CdSe may exhibit relatively strong resistance to oxidation compared to PbSe, CdSe was chosen as a shell candidate to overcome the limited chemical stability of PbSe.77 In addition, bulk CdSe shows very close lattice matching (mismatch of ≤1%)77-78 with bulk PbSe (6.08 and 6.12 Å for zinc blend CdSe and rock-salt PbSe, respectively).79-80 However, using conventional co-precipitation and SILAR approaches to produce PbSe/CdSe core/shell NCs is difficult because Ostwald ripening of PbSe core NCs occurs at a high temperature that is required for the CdSe shell coating. In consequence, the cation exchange was proposed as an alternative approach.75, 81-83 Figure 3A and 3B display high-angle annular dark field high resolution scanning transmission electron microscope (HAADF-HRSTEM) images of PbSe/CdSe core/shell NCs taken after 30 min and 60 min into the exchange reaction. The inset displays a magnified image of a single NC, presenting {111} set of facets by yellow lines. These images indicate a distinct contrast between the core and the shell due to the large atomic weight difference between Pb and Cd ions. A nonconcentric alignment of the shell with respect to the NC center was observed after 30 min into the exchange process, as shown in Figure 3A. But after 60 min of reaction, a uniform shell covering the inner PbSe core was formed (Figure 3B). Careful investigation revealed that the cation exchange is initiated by replacing Cd with Pb atoms at (111) strained corners, and it sequentially proceeds in the ‹111› direction.81 Because small-sized NCs exhibit truncated cubic corners, a cation exchange might start only from one side of the NC, encouraging nonconcentric growth of the shell with respect to the center of the core.


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Figure 3C displays XRD patterns of the PbSe core (curve (i)) and PbSe/CdSe core/shell NCs (curves (ii) and (iii)), obtained after 30 and 60 min of cation exchange. In the XRD pattern of the PbSe core NCs, the (202) peak which is referred to the Pb3O4 phase was observed due to oxidation, while the intensity of this peak decreased after 30 min (curve (ii)) and was totally absent after 60 min (curve (iii)), showing improved chemical stability by the present of the CdSe shell. The main peaks of the PbSe/CdSe core/shell NCs shift towards the CdSe phase, verifying the CdSe shell formation. Figure 3D exhibits the absorption (dashed lines) and PL (bold lines) spectra of the PbSe core NCs and of the corresponding PbSe/CdSe core/shell NCs obtained at different reaction stages. An initial blue-shift of the first absorption peak with respect to that of the PbSe cores is observed until 20 min, as a result of reduction of the core size. But, as the reaction progressed, the spectral shift was detained, and after prolongation of the reaction (~25 min), a retracting red-shift of ~30 meV was observed.

Figure 3. Representative HAADF-STEM images of PbSe/CdSe core/shell NCs taken at (A) 30 min and (B) 60 min. The dashed box outlines a magnified image of a selected NC. (C) XRD patterns showing (i) PbSe core and PbSe/CdSe core/shell NCs after (ii) 30 min and (iii) 60 min


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of exchange reaction. (D) Absorption (dashed lines) and PL (bold lines) spectra of PbSe core and PbSe/CdSe core/shell NCs. Reproduced from ref. 75. Copyright 2015 American Chemical Society.

In spite of the intriguing properties of Pb chalcogenide NCs, offering wide implementation in diverse technological applications, there is a demand to find an alternative material, owing to the toxicity of the Pb element to the environment. Recently, the Lifshitz group achieved SnTe/CdTe NCs using two steps, consisting of synthesis of SnTe NCs, followed by a cation exchange of Sn2+ by Cd2+.44 Further details are given in our previous report.44 Figure 4A and 4B present HRTEM images of SnTe core NCs with 14.5± 0.8 nm and the corresponding SnTe/CdTe core/shell NCs. The total size of SnTe/CdTe core/shell NCs was measured to be 14.5 ± 1.0 nm, indicating that the exterior size of the NCs is maintained, supporting the exchange process from the outer to inner parts of the NCs. Figure 4B clearly exhibits a core/shell configuration, with pronounced lattice fringes at the outer ring spaced by 0.374 nm, corresponding to the (111) planes of a CdTe shell. To illustrate precisely the elemental distribution across the NC, a HAADF-STEM analysis with energy dispersive X-ray (EDX) spectroscopy was performed. Figure 4C shows a representative example, indicating that most of the Cd atoms are present in the shell region, while the core is rich with the Sn element. This analysis validates the formation of SnTe/CdTe core/shell NCs. Figure 4D presents the X-ray photoelectron spectroscopy (XPS) spectra of the SnTe core and the SnTe/CdTe core/shell NCs, indicating that the Te4+ peaks were clearly detected in the SnTe core NCs (top), while the signatures of Te4+ nearly disappeared after shell coating (bottom), verifying an effective surface passivation by a CdTe shell.


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Figure 4. HRTEM images of (A) the SnTe core and (B) SnTe/CdTe core/shell NCs after the cation exchange. (C) HAADF-STEM line profile of a SnTe/CdTe core/shell NC. (D) Te 3d XPS spectra of the SnTe core (top) and SnTe/CdTe core/shell NCs (bottom). Reproduced from ref. 44. Copyright 2016 American Chemical Society.

4. Kirkendall effect In the 1940s, Kirkendall and co-workers demonstrated an unequal diffusion rate between two components,84-86 the so-called Kirkendall effect. In bulk materials, this event is considered undesirable because of the formation of voids, resulting in degradation of the mechanical properties. On the contrary, the Kirkendall effect can offer wide opportunities for the preparation of core/shell NHs. In 2004, Alivisatos and co-workers implemented this effect to tailor hollowed nanostructures.87 This observation was followed by a few other groups for the synthesis of core/shell NHs, displaying cases in which cation diffusion outward exceeded the inward move of the replacing cation.88-93


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Recently, a unique case is introduced, in which diffusion inward happened was faster than that outward, leading to the formation of inverted core/shell NCs.44, 94 An example given here is related to the preparation of SnTe-based core/shell NHs by applying another semiconductors (e.g., CdTe) with close crystallographic matching between constituents. A conversion from SnTe/CdTe core/shell NCs to CdTe/SnTe core/shell NCs was reported by the Lifshitz group.44 Core/shell of the kind SnTe/PbTe (not shown here) was created recently, following a conventional Kirkendall effect with a larger diffusion rate in the outward direction. This work will be published soon elsewhere. HRTEM image of a single core/shell NC withdrawn from the reaction vessel after 20 min is presented in Figure 5A. The image shows the appearance of semi-ordered domains; an external ring, as well as narrow bridges connecting the center to an outer ring (see the area framed by the yellow lines) is composed of SnTe fringes, while other areas are dominated by CdTe planes (see the area framed by the pink line). Approximately 79% of the NCs showed bridge signatures in the HRTEM images of the intermediate stage. Figure 5B displays the HAADF-STEM line scan image, also indicating the rearrangement of elemental (Sn and Cd) distributions in the NC. The observations imply migration of the Sn-cation via the bridges toward the exterior shell and Cd cation motion toward the inner core. After prolonged reaction time (>1 h), the dramatic phenomenon occurred, as seen in Figure 5C. A continuation of the reaction eventually generates complete penetration of the Cd-cations into the core area, and at the same time, the Sn-cations occupied the shell regime, resulting in the formation of CdTe/SnTe inverted core/shell NCs. The elemental line profile by HAADF-STEM analysis of the inverted core/shell NC is represented in Figure 5D, confirming a shell and a core dominated by Sn and Cd atoms, respectively.


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Figure 5. (A and C) HRTEM images and (B and D) HAADF-STEM line profiles of intermediate SnTe-CdTe and CdTe/SnTe core/shell NC taken after different reaction times. (A and B: 20 min; C and D: 70 min). Reproduced from ref. 44. Copyright 2016 American Chemical Society.

The formation process was further corroborated by a theoretical calculation, employing a diffusion coefficient (D) and diffusion distance (L). Those values can be estimated by using an Arrhenius relation (eq 1) and the root-mean-square distance (eq 2), respectively:95-97

(1)

D0 is the pre-exponential factor, E is the activation energy and T is temperature. √2

(2)

where t is time. Accordingly, the diffusion coefficients of Sn and Cd in the procedure were calculated to be ~3.4 × 102 and ~8.2 × 103 nm2 s−1, respectively. In addition, the anticipated diffusion distances of Sn and Cd ions were found to be ~2 × 102 and ~1 × 103 nm after 1 min at the reaction temperature, respectively, indicating distances that are much larger than the typical


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size of the investigated NCs. The control experiments showed that an extensive diffusion process actually was ignited around the reaction temperature of 180 °C, correlated with a temperature dependence which is dictated by the Arrhenius relation (eq 1).98-99 Few sub-mechanisms were proposed to describe Kirkendall effect on a sub-micron level: (a) Direct vacancy mechanism, where vacancies are displaced from one crystal side to another; (b) kick-out mechanism,100-102 in which foreign impurities occupy interstitial positions, moving from one to another and eventually displacing a lattice atom. Consequently, the dislodged host atoms may become selfinterstitial while the impurity atoms become immobile; (c) Frank-Turnbull mechanism,102-103 involving free motion of impurities from one interstitial side to another until they get trapped at vacancy positions, whereupon they are almost immobile.

5. Summary and perspectives Semiconductor core/shell NHs possess many benefits compared to pure cores, exhibiting excellent applicability in various fields such as bio-imaging, photovoltaics, chemical sensors and light-emitting diodes. The present article described recent progress in the preparation of IV-VI semiconductor core/shell NCs. The co-precipitation method has been widely used in core/shell synthesis, but it usually needs high temperatures for the shell coating, which involves limitations when using core NCs having low thermal stability because of the occurrence of Ostwald ripening. Therefore, in addition to the conventional co-precipitation method, another approach should be considered. Cation exchange and the Kirkendall effect can be alternative routes, providing a breakthrough for the synthesis of core/shell NCs. In the two processes, low temperature is usually applied, in


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contrast to conventional routes such as co-precipitation or the SILAR approach. Recent exciting discoveries for the preparation of IV-VI core/shell NCs were discussed in this article. In spite of the advantages of the two synthetic routes, challenges still remain. First, a choice of the shell material is less flexible in binary semiconductors. In general, the cation exchange and Kirkendall effect involve a substitution of one element (e.g., metal cation or chalcogenide anion) in a material. It suggests that two sequential steps are required to cover one semiconductor core with another material consisting of different elements (e.g., CdSe/ZnS), whereas CdSe/ZnS core/shell NCs are readily obtained by the conventional approach (e.g., co-precipitation or SILAR). Moreover, in spite of some reports,104-107 substitution of anions is considered to be difficult because the anions are less mobile than the cations, since the anions generally have a larger ionic radius and a slower diffusion rate than the cations. Second, a lattice defect can be formed during the process, resulting in poor optical properties.108 The occurrence of stacking faults or two different crystal domains' boundaries after cation exchange process was reported,67, 109

although it was not observed in same semiconductor NCs prepared by the co-precipitation

method. Last, to date, a detailed formation process is not fully explored, despite the fact that several sub-mechanisms have been suggested. It is difficult to describe a mechanism because the exchange or diffusion process is dynamic action and is affected by many factors (e.g., vacancy, stacking fault and defects inside the NC). In addition, more theoretical studies are required to acquire a deep understanding of the processes and elucidate the observations. Close collaboration between theoretical and experimental endeavors allows us to produce newly tailored core/shell NHs possessing unique properties and superior performance in various application fields.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions †These authors contributed equally. Notes The authors declare no competing financial interest.

Acknowledgments We thank G. Zaiats for his contribution in the past. This work was supported by the Israel Council for Higher Education-Focal Area Technology (Project No. 872967), the Volkswagen Stiftung (Project No. 88116), the Israel Ministry of Defense (Project No. 4440827018), the Israel Ministry of Trade (Maymad Project No. 54662), the Israel Science Foundation Bikura (Project No. 1508/14), the Israel Science Foundation (Project No. 985/11 and 914/15), the Niedersachsen-Deutsche Technion Gesellschaft E.V. (Project No. ZN2916) and the European Commission via the Marie-Sklodowska Curie Action Phonsi (Project No. H2020-MSCAITN642656).


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