Shell Heterostructured

Feb 23, 2018 - Colloidal nanocrystals (NCs) have been at the forefront of scientific research and technological applications since their emergence in ...
0 downloads 3 Views 1MB Size
Subscriber access provided by - Access paid by the | UCSB Libraries

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

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.

ACS Paragon Plus Environment

2

Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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.

ACS Paragon Plus Environment

3

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

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

ACS Paragon Plus Environment

4

Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(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.

ACS Paragon Plus Environment

5

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

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

ACS Paragon Plus Environment

6

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

7

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

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

ACS Paragon Plus Environment

8

Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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.

ACS Paragon Plus Environment

9

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

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

ACS Paragon Plus Environment

10

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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.

ACS Paragon Plus Environment

11

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

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

ACS Paragon Plus Environment

12

Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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.

ACS Paragon Plus Environment

13

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

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

ACS Paragon Plus Environment

14

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

15

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

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.

ACS Paragon Plus Environment

16

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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).

ACS Paragon Plus Environment

17

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

References (1)

Murray, C. B.; Kagan, C. R.; Bawendi, M. G., Synthesis and Characterization of

Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies. Annu. Rev. Mater. Sci. 2000, 30, 545-610. (2)

Vanmaekelbergh, D.; Liljeroth, P., Electron-Conducting Quantum Dot Solids: Novel

Materials Based on Colloidal Semiconductor Nanocrystals. Chem. Soc. Rev. 2005, 34, 299-312. (3)

Kambhampati, P., Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots:

Radiationless Transitions on the Nanoscale. J. Phys. Chem. C 2011, 115, 22089-22109. (4)

Kovalenko, M. V., et al., Prospects of Nanoscience with Nanocrystals. ACS Nano 2015,

9, 1012-1057. (5)

Kagan, C. R.; Lifshitz, E.; Sargent, E. H.; Talapin, D. V., Building Devices from

Colloidal Quantum Dots. Science 2016, 353, aac5523. (6)

Owen, J., Nanocrystal Structure. The Coordination Chemistry of Nanocrystal Surfaces.

Science 2015, 347, 615-616. (7)

Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S., Ligand Exchange and the

Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile MetalCarboxylate Displacement and Binding. J. Am. Chem. Soc. 2013, 135, 18536-18548. (8)

Manna, L.; Wang, L. W.; Cingolani, R.; Alivisatos, A. P., First-Principles Modeling of

Unpassivated and Surfactant-Passivated Bulk Facets of Wurtzite CdSe: A Model System for Studying the Anisotropic Growth of CdSe Nanocrystals. J. Phys. Chem. B 2005, 109, 6183-6192.

ACS Paragon Plus Environment

18

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(9)

Choi, H.; Ko, J. H.; Kim, Y. H.; Jeong, S., Steric-Hindrance-Driven Shape Transition in

PbS Quantum Dots: Understanding Size-Dependent Stability. J. Am. Chem. Soc. 2013, 135, 5278-5281. (10) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P., Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019-7029. (11) Kershaw, S. V.; Burt, M.; Harrison, M.; Rogach, A.; Weller, H.; Eychmüller, A., Colloidal CdTe/HgTe Quantum Dots with High Photoluminescence Quantum Efficiency at Room Temperature. Appl. Phys. Lett. 1999, 75, 1694-1696. (12) Cao, Y.-W.; Banin, U., Synthesis and Characterization of InAs/InP and InAs/CdSe Core/Shell Nanocrystals. Angew. Chem., Int. Ed. 1999, 38, 3692-3694. (13) Reiss, P.; Bleuse, J.; Pron, A., Highly Luminescent CdSe/ZnSe Core/Shell Nanocrystals of Low Size Dispersion. Nano Lett. 2002, 2, 781-784. (14) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G., Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air-Stable Reagents Via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125, 12567-12575. (15) Aharoni, A.; Mokari, T.; Popov, I.; Banin, U., Synthesis of InAs/CdSe/ZnSe Core/Shell1/Shell2 Structures with Bright and Stable Near-Infrared Fluorescence. J. Am. Chem. Soc. 2005, 128, 257-264.

ACS Paragon Plus Environment

19

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

(16) Guo, Y.; Rowland, C. E.; Schaller, R. D.; Vela, J., Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II-VI Semiconductor Epitaxy. ACS Nano 2014, 8, 8334-8343. (17) Boote, B. W.; Men, L.; Andaraarachchi, H. P.; Bhattacharjee, U.; Petrich, J. W.; Vela, J.; Smith, E. A., Germanium-Tin/Cadmium Sulfide Core/Shell Nanocrystals with Enhanced nearInfrared Photoluminescence. Chem. Mater. 2017, 29, 6012-6021. (18) Shimizu, K. T.; Woo, W. K.; Fisher, B. R.; Eisler, H. J.; Bawendi, M. G., SurfaceEnhanced Emission from Single Semiconductor Nanocrystals. Phys. Rev. Lett. 2002, 89, 117401. (19) Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.; Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A., "Giant" Multishell CdSe Nanocrystal Quantum Dots with Suppressed Blinking. J. Am. Chem. Soc. 2008, 130, 5026-5027. (20) Mahler, B.; Spinicelli, P.; Buil, S.; Quelin, X.; Hermier, J.-P.; Dubertret, B., Towards Non-Blinking Colloidal Quantum Dots. Nat. Mater. 2008, 7, 659-664. (21) Dennis, A. M.; Mangum, B. D.; Piryatinski, A.; Park, Y.-S.; Hannah, D. C.; Casson, J. L.; Williams, D. J.; Schaller, R. D.; Htoon, H.; Hollingsworth, J. A., Suppressed Blinking and Auger Recombination in Near-Infrared Type-II InP/CdS Nanocrystal Quantum Dots. Nano Lett. 2012, 12, 5545-5551. (22) Cozzoli, P. D.; Pellegrino, T.; Manna, L., Synthesis, Properties and Perspectives of Hybrid Nanocrystal Structures. Chem. Soc. Rev. 2006, 35, 1195-1208. (23) Reiss, P.; Protière, M.; Li, L., Core/Shell Semiconductor Nanocrystals. Small 2009, 5, 154-168.

ACS Paragon Plus Environment

20

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(24) Smith, A. M.; Nie, S. M., Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering. Acc. Chem. Res. 2010, 43, 190-200. (25) Costi, R.; Saunders, A. E.; Banin, U., Colloidal Hybrid Nanostructures: A New Type of Functional Materials. Angew. Chem., Int. Ed. 2010, 49, 4878-4897. (26) Donega, C. d. M., Synthesis and Properties of Colloidal Heteronanocrystals. Chem. Soc. Rev. 2011, 40, 1512-1546. (27) Kundu, P.; Anumol, E. A.; Nethravathi, C.; Ravishankar, N., Existing and Emerging Strategies for the Synthesis of Nanoscale Heterostructures. Phys. Chem. Chem. Phys. 2011, 13, 19256-19269. (28) Ghosh Chaudhuri, R.; Paria, S., Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373-2433. (29) Wang, H.; Chen, L.; Feng, Y.; Chen, H., Exploiting Core-Shell Synergy for Nanosynthesis and Mechanistic Investigation. Acc. Chem. Res. 2013, 46, 1636-1646. (30) Hines, D. A.; Kamat, P. V., Recent Advances in Quantum Dot Surface Chemistry. ACS Appl. Mater. Interfaces 2014, 6, 3041-3057. (31) Kim, M. R.; Xu, Z.; Chen, G.; Ma, D., Semiconductor and Metallic Core-Shell Nanostructures: Synthesis and Applications in Solar Cells and Catalysis. Chem. Eur. J. 2014, 20, 11256-11275. (32) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G., (CdSe)ZnS Core-Shell Quantum Dots: Synthesis and

ACS Paragon Plus Environment

21

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

Characterization of a Size Series of Highly Luminescent Nanocrystallites. J. Phys. Chem. B 1997, 101, 9463-9475. (33) Hines, M. A.; Guyot-Sionnest, P., Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals. J. Phys. Chem. 1996, 100, 468-471. (34) Haubold, S.; Haase, M.; Kornowski, A.; Weller, H., Strongly Luminescent InP/ZnS CoreShell Nanoparticles. ChemPhysChem 2001, 2, 331-334. (35) Mekis, I.; Talapin, D. V.; Kornowski, A.; Haase, M.; Weller, H., One-Pot Synthesis of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals Via Organometallic and "Greener" Chemical Approaches. J. Phys. Chem. B 2003, 107, 7454-7462. (36) Lifshitz, E., et al., Air-Stable PbSe/PbS and PbSe/PbSexS1-x Core/Shell Nanocrystal Quantum Dots and Their Applications. J. Phys. Chem. B 2006, 110, 25356-25365. (37) Deng, Z.; Guyot-Sionnest, P., Intraband Luminescence from HgSe/CdS Core/Shell Quantum Dots. ACS Nano 2016, 10, 2121-2127. (38) Zeng, Q.; Kong, X.; Sun, Y.; Zhang, Y.; Tu, L.; Zhao, J.; Zhang, H., Synthesis and Optical Properties of Type II CdTe/CdS Core/Shell Quantum Dots in Aqueous Solution Via Successive Ion Layer Adsorption and Reaction. J. Phys. Chem. C 2008, 112, 8587-8593. (39) Narayanaswamy, A.; Feiner, L. F.; Meijerink, A.; van der Zaag, P. J., The Effect of Temperature and Dot Size on the Spectral Properties of Colloidal InP/ZnS Core-Shell Quantum Dots. ACS Nano 2009, 3, 2539-2546. (40) Baghbanzadeh, M.; Carbone, L.; Cozzoli, P. D.; Kappe, C. O., Microwave-Assisted Synthesis of Colloidal Inorganic Nanocrystals. Angew. Chem., Int. Ed. 2011, 50, 11312-11359.

ACS Paragon Plus Environment

22

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(41) He, Y.; Lu, H. T.; Sai, L. M.; Lai, W. Y.; Fan, Q. L.; Wang, L. H.; Huang, W., Microwave-Assisted Growth and Characterization of Water-Dispersed CdTe/CdS Core-Shell Nanocrystals with High Photoluminescence. J. Phys. Chem. B 2006, 110, 13370-13374. (42) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E., Nucleation and Growth of Cadmium Selendie on Zinc Sulfide Quantum Crystallite Seeds, and Vice Versa, in Inverse Micelle Media. J. Am. Chem. Soc. 1990, 112, 1327-1332. (43) Hao, E.; Sun, H.; Zhou, Z.; Liu, J.; Yang, B.; Shen, J., Synthesis and Optical Properties of CdSe and CdSe/CdS Nanoparticles. Chem. Mater. 1999, 11, 3096-3102. (44) Jang, Y.; Yanover, D.; Capek, R. K.; Shapiro, A.; Grumbach, N.; Kauffmann, Y.; Sashchiuk, A.; Lifshitz, E., Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals. J. Phys. Chem. Lett. 2016, 7, 26022609. (45) Kim, S.; Marshall, A. R.; Kroupa, D. M.; Miller, E. M.; Luther, J. M.; Jeong, S.; Beard, M. C., Air-Stable and Efficient Pbse Quantum-Dot Solar Cells Based Upon ZnSe to PbSe Cation-Exchanged Quantum Dots. ACS Nano 2015, 9, 8157-8164. (46) Park, J.; Kim, S.-W., CuInS2/ZnS Core/Shell Quantum Dots by Cation Exchange and Their Blue-Shifted Photoluminescence. J. Mater. Chem. 2011, 21, 3745-3750. (47) Lambert, K.; Geyter, B. D.; Moreels, I.; Hens, Z., PbTe|CdTe Core|Shell Particles by Cation Exchange, a HR-TEM Study. Chem. Mater. 2009, 21, 778-780. (48) Cao; Banin, U., Growth and Properties of Semiconductor Core/Shell Nanocrystals with InAs Cores. J. Am. Chem. Soc. 2000, 122, 9692-9702.

ACS Paragon Plus Environment

23

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

(49) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H., Highly Luminescent

Monodisperse

CdSe

and

CdSe/ZnS

Nanocrystals

Synthesized

in

a

Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture. Nano Lett. 2001, 1, 207211. (50) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M., Epitaxial Core-Shell and CoreMultishell Nanowire Heterostructures. Nature 2002, 420, 57-61. (51) Li, L.; Reiss, P., One-Pot Synthesis of Highly Luminescent InP/ZnS Nanocrystals without Precursor Injection. J. Am. Chem. Soc. 2008, 130, 11588-11589. (52) Tessier, M. D.; Dupont, D.; De Nolf, K.; De Roo, J.; Hens, Z., Economic and SizeTunable Synthesis of InP/ZnE (E = S, Se) Colloidal Quantum Dots. Chem. Mater. 2015, 27, 4893-4898. (53) Danek, M.; Jensen, K. F.; Murray, C. B.; Bawendi, M. G., Synthesis of Luminescent Thin-Film CdSe/ZnSe Quantum Dot Composites Using Cdse Quantum Dots Passivated with an Overlayer of ZnSe. Chem. Mater. 1996, 8, 173-180. (54) Talapin, D. V.; Mekis, I.; Götzinger, S.; Kornowski, A.; Benson, O.; Weller, H., CdSe/CdS/ZnS and CdSe/ZnSe/ZnS Core-Shell-Shell Nanocrystals. J. Phys. Chem. B 2004, 108, 18826-18831. (55) Harrison, M. T.; Kershaw, S. V.; Rogach, A. L.; Kornowski, A.; Eychmüller, A.; Weller, H., Wet Chemical Synthesis of Highly Luminescent HgTe/CdS Core/Shell Nanocrystals. Adv. Mater. 2000, 12, 123-125.

ACS Paragon Plus Environment

24

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(56) Protiere, M.; Reiss, P., Highly Luminescent Cd1-xZnxSe/ZnS Core/Shell Nanocrystals Emitting in the Blue-Green Spectral Range. Small 2007, 3, 399-403. (57) Chen, O., et al., Compact High-Quality CdSe-CdS Core-Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12, 445-451. (58) Cirillo, M.; Aubert, T.; Gomes, R.; Van Deun, R.; Emplit, P.; Biermann, A.; Lange, H.; Thomsen, C.; Brainis, E.; Hens, Z., "Flash" Synthesis of CdSe/CdS Core-Shell Quantum Dots. Chem. Mater. 2014, 26, 1154-1160. (59) Grumbach, N.; Capek, R. K.; Tilchin, E.; Rubin-Brusilovski, A.; Yang, J.; Ein-Eli, Y.; Lifshitz, E., Comprehensive Route to the Formation of Alloy Interface in Core/Shell Colloidal Quantum Dots. J. Phys. Chem. C 2015, 119, 12749-12756. (60) Sashchiuk, A.; Langof, L.; Chaim, R.; Lifshitz, E., Synthesis and Characterization of PbSe and PbSe/PbS Core-Shell Colloidal Nanocrystals. J. Cryst. Growth 2002, 240, 431-438. (61) Yanover, D.; Čapek, R. K.; Rubin-Brusilovski, A.; Vaxenburg, R.; Grumbach, N.; Maikov, G. I.; Solomeshch, O.; Sashchiuk, A.; Lifshitz, E., Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots. Chem. Mater. 2012, 24, 4417-4423. (62) Shapiro, A.; Jang, Y.; Rubin-Brusilovski, A.; Budniak, A. K.; Horani, F.; Sashchiuk, A.; Lifshitz, E., Tuning Optical Activity of IV-VI Colloidal Quantum Dots in the Short-Wave Infrared (SWIR) Spectral Regime. Chem. Mater. 2016, 28, 6409-6416. (63) Choi, H.; Song, J. H.; Jang, J.; Mai, X. D.; Kim, S.; Jeong, S., High Performance of PbSe/PbS Core/Shell Quantum Dot Heterojunction Solar Cells: Short Circuit Current

ACS Paragon Plus Environment

25

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

Enhancement without the Loss of Open Circuit Voltage by Shell Thickness Control. Nanoscale 2015, 7, 17473-17481. (64) Brumer, M.; Kigel, A.; Amirav, L.; Sashchiuk, A.; Solomesch, O.; Tessler, N.; Lifshitz, E., PbSe/PbS and PbSe/PbSexS1-x Core/Shell Nanocrystals. Adv. Funct. Mater. 2005, 15, 11111116. (65) Schooss, D.; Mews, A.; Eychmuller, A.; Weller, H., Quantum-Dot Quantum Well CdS/HgS/CdS: Theory and Experiment. Phys. Rev. B 1994, 49, 17072-17078. (66) Zhou, H. S.; Sasahara, H.; Honma, I.; Komiyama, H.; Haus, J. W., Coated Semiconductor Nanoparticles: The CdS/PbS System's Photoluminescence Properties. Chem. Mater. 1994, 6, 1534-1541. (67) Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P., Cation Exchange Reactions in Ionic Nanocrystals. Science 2004, 306, 1009-1012. (68) Min, Y.; Moon, G. D.; Park, J.; Park, M.; Jeong, U., Surfactant-Free CuInSe2 Nanocrystals Transformed from In2Se3 Nanoparticles and Their Application for a Flexible UV Photodetector. Nanotechnology 2011, 22, 465604. (69) Xie, Y.; Bertoni, G.; Riedinger, A.; Sathya, A.; Prato, M.; Marras, S.; Tu, R.; Pellegrino, T.; Manna, L., Nanoscale Transformations in Covellite (CuS) Nanocrystals in the Presence of Divalent Metal Cations in a Mild Reducing Environment. Chem. Mater. 2015, 27, 7531-7537. (70) Gupta, S.; Kershaw, S. V.; Rogach, A. L., 25th Anniversary Article: Ion Exchange in Colloidal Nanocrystals. Adv. Mater. 2013, 25, 6923-6943.

ACS Paragon Plus Environment

26

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(71) De Trizio, L.; Manna, L., Forging Colloidal Nanostructures Via Cation Exchange Reactions. Chem. Rev. 2016, 116, 10852-10887. (72) Rivest, J. B.; Jain, P. K., Cation Exchange on the Nanoscale: An Emerging Technique for New Material Synthesis, Device Fabrication, and Chemical Sensing. Chem. Soc. Rev. 2013, 42, 89-96. (73) Moon, G. D.; Ko, S.; Min, Y.; Zeng, J.; Xia, Y.; Jeong, U., Chemical Transformations of Nanostructured Materials. Nano Today 2011, 6, 186-203. (74) Beberwyck, B. J.; Surendranath, Y.; Alivisatos, A. P., Cation Exchange: A Versatile Tool for Nanomaterials Synthesis. J. Phys. Chem. C 2013, 117, 19759-19770. (75) Zaiats, G.; Shapiro, A.; Yanover, D.; Kauffmann, Y.; Sashchiuk, A.; Lifshitz, E., Optical and Electronic Properties of Nonconcentric PbSe/CdSe Colloidal Quantum Dots. J. Phys. Chem. Lett. 2015, 6, 2444-2448. (76) Zaiats, G.; Yanover, D.; Vaxenburg, R.; Shapiro, A.; Safran, A.; Hesseg, I.; Sashchiuk, A.; Lifshitz, E., PbSe/CdSe Thin-Shell Colloidal Quantum Dots. Z. Phys. Chem. 2015, 229, 3-21. (77) Pietryga, J. M.; Werder, D. J.; Williams, D. J.; Casson, J. L.; Schaller, R. D.; Klimov, V. I.; Hollingsworth, J. A., Utilizing the Lability of Lead Selenide to Produce Heterostructured Nanocrystals with Bright, Stable Infrared Emission. J. Am. Chem. Soc. 2008, 130, 4879-4885. (78) Grodzińska, D.; Evers, W. H.; Dorland, R.; van Rijssel, J.; van Huis, M. A.; Meijerink, A.; de Mello Donegá, C.; Vanmaekelbergh, D., Two-Fold Emission from the S-Shell of PbSe/CdSe Core/Shell Quantum Dots. Small 2011, 7, 3493-3501.

ACS Paragon Plus Environment

27

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

(79) Samarth, N.; Luo, H.; Furdyna, J. K.; Qadri, S. B.; Lee, Y. R.; Ramdas, A. K.; Otsuka, N., Growth of Cubic (Zinc Blende) CdSe by Molecular Beam Epitaxy. Appl. Phys. Lett. 1989, 54, 2680. (80) Fan, Z.; Koster, R. S.; Wang, S.; Fang, C.; Yalcin, A. O.; Tichelaar, F. D.; Zandbergen, H. W.; van Huis, M. A.; Vlugt, T. J., A Transferable Force Field for CdS-CdSe-PbS-PbSe Solid Systems. J. Chem. Phys. 2014, 141, 244503. (81) Casavola, M.; van Huis, M. A.; Bals, S.; Lambert, K.; Hens, Z.; Vanmaekelbergh, D., Anisotropic Cation Exchange in PbSe/CdSe Core/Shell Nanocrystals of Different Geometry. Chem. Mater. 2012, 24, 294-302. (82) Abel, K. A.; Qiao, H.; Young, J. F.; van Veggel, F. C. J. M., Four-Fold Enhancement of the Activation Energy for Nonradiative Decay of Excitons in PbSe/CdSe Core/Shell versus PbSe Colloidal Quantum Dots. J. Phys. Chem. Lett. 2010, 1, 2334-2338. (83) De Geyter, B., et al., The Different Nature of Band Edge Absorption and Emission in Colloidal PbSe/CdSe Core/Shell Quantum Dots. ACS Nano 2011, 5, 58-66. (84) Kirkendall, E.; Thomassen, L.; Upthegrove, C., Rates of Diffusion of Copper and Zinc in Alpha Brass. Trans. Am. Inst. Min. Metall. 1939, 133, 186-203. (85) Kirkendall, E. O., Diffusion of Zinc in Alpha Brass. Trans. Am. Inst. Min. Metall. 1942, 147, 104-110. (86) Smigelskas, A. D.; Kirkendall, E. O., Zinc Diffusion in Alpha Brass. Trans. Am. Inst. Min. Metall. 1947, 171, 130-142.

ACS Paragon Plus Environment

28

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(87) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P., Formation of Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Science 2004, 304, 711-714. (88) Wang, W.; Dahl, M.; Yin, Y., Hollow Nanocrystals through the Nanoscale Kirkendall Effect. Chem. Mater. 2013, 25, 1179-1189. (89) Yin, Y.; Erdonmez, C. K.; Cabot, A.; Hughes, S.; Alivisatos, A. P., Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals. Adv. Funct. Mater. 2006, 16, 1389-1399. (90) Yu, Y.; Yin, X.; Kvit, A.; Wang, X., Evolution of Hollow TiO2 Nanostructures Via the Kirkendall Effect Driven by Cation Exchange with Enhanced Photoelectrochemical Performance. Nano Lett. 2014, 14, 2528-2535. (91) Dong, C.; Yao, D.; Feng, J.; Huang, T.; Hu, X.; Wu, Z.; Liu, Y.; Yang, B.; Zhang, H., Facile Synthesis of Cu2GeS3 and Cu2MgeS4 (M = Zn, Mn, Fe, Co, and Ni) Hollow Nanoparticles, Based on the Nanoscale Kirkendall Effect. Chem. Mater. 2016, 28, 9139-9149. (92) Fan, H. J.; Knez, M.; Scholz, R.; Hesse, D.; Nielsch, K.; Zacharias, M.; Gosele, U., Influence of Surface Diffusion on the Formation of Hollow Nanostructures Induced by the Kirkendall Effect: The Basic Concept. Nano Lett. 2007, 7, 993-997. (93) Niu, K. Y.; Park, J.; Zheng, H.; Alivisatos, A. P., Revealing Bismuth Oxide Hollow Nanoparticle Formation by the Kirkendall Effect. Nano Lett. 2013, 13, 5715-9. (94) Liu, J.; Amit, Y.; Li, Y.; Plonka, A. M.; Ghose, S.; Zhang, L.; Stach, E. A.; Banin, U.; Frenkel, A. I., Reversed Nanoscale Kirkendall Effect in Au-InAs Hybrid Nanoparticles. Chem. Mater. 2016, 28, 8032-8043.

ACS Paragon Plus Environment

29

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

(95) Mehrer, H.; Stolica, N.; Stolwijk, N., Diffusion in Solid Metals and Alloys; Springer Berlin Heidelberg, 1990. (96) Mokari, T.; Aharoni, A.; Popov, I.; Banin, U., Diffusion of Gold into InAs Nanocrystals. Angew. Chem., Int. Ed. 2006, 45, 8001-8005. (97) Cabot, A.; Smith, R. K.; Yin, Y.; Zheng, H.; Reinhard, B. M.; Liu, H.; Alivisatos, A. P., Sulfidation of Cadmium at the Nanoscale. ACS Nano 2008, 2, 1452-1458. (98) Groeneveld, E.; Witteman, L.; Lefferts, M.; Ke, X.; Bals, S.; Van Tendeloo, G.; Donega Cde, M., Tailoring ZnSe-CdSe Colloidal Quantum Dots Via Cation Exchange: From Core/Shell to Alloy Nanocrystals. ACS Nano 2013, 7, 7913-7930. (99) Wang, C.; Peng, S.; Chan, R.; Sun, S., Synthesis of Auag Alloy Nanoparticles from Core/Shell-Structured Ag/Au. Small 2009, 5, 567-570. (100) Gösele, U.; Frank, W.; Seeger, A., Mechanism and Kinetics of the Diffusion of Gold in Silicon. Appl. Phys. 1980, 23, 361-368. (101) Gösele, U.; Morehead, F.; Frank, W.; Seeger, A., Diffusion of Gold in Silicon: A New Model. Appl. Phys. Lett. 1981, 38, 157-159. (102) Gösele, U. M., Fast Diffusion in Semiconductors. Annu. Rev. Mater. Sci. 1988, 18, 257282. (103) Frank, F. C.; Turnbull, D., Mechanism of Diffusion of Copper in Germanium. Phys. Rev. 1956, 104, 617-618.

ACS Paragon Plus Environment

30

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(104) Zhao, W.; Zhang, C.; Geng, F.; Zhuo, S.; Zhang, B., Nanoporous Hollow Transition Metal Chalcogenide Nanosheets Synthesized Via the Anion-Exchange Reaction of Metal Hydroxides with Chalcogenide Ions. ACS Nano 2014, 8, 10909-10919. (105) Langevin, M. A.; Ritcey, A. M.; Allen, C. N., Air-Stable Near-Infrared AgInSe2 Nanocrystals. ACS Nano 2014, 8, 3476-3482. (106) Saruyama, M.; So, Y. G.; Kimoto, K.; Taguchi, S.; Kanemitsu, Y.; Teranishi, T., Spontaneous Formation of Wurzite-CdS/Zinc Blende-CdTe Heterodimers through a Partial Anion Exchange Reaction. J. Am. Chem. Soc. 2011, 133, 17598-601. (107) Park, J.; Zheng, H.; Jun, Y. W.; Alivisatos, A. P., Hetero-Epitaxial Anion Exchange Yields Single-Crystalline Hollow Nanoparticles. J. Am. Chem. Soc. 2009, 131, 13943-13945. (108) Shamsi, J.; Dang, Z.; Ijaz, P.; Abdelhady, A. L.; Bertoni, G.; Moreels, I.; Manna, L., Colloidal CsX (X = Cl, Br, I) Nanocrystals and Their Transformation to CsPbX3 Nanocrystals by Cation Exchange. Chem. Mater. 2017, 30, 79-83. (109) Jain, P. K.; Beberwyck, B. J.; Fong, L. K.; Polking, M. J.; Alivisatos, A. P., Highly Luminescent Nanocrystals from Removal of Impurity Atoms Residual from Ion-Exchange Synthesis. Angew. Chem., Int. Ed. 2012, 51, 2387-2390.

ACS Paragon Plus Environment

31

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

TOC Graphic

ACS Paragon Plus Environment

32