Following Transformations at Nanoscale

Mechanism of Ion Exchange in CdSe/Cu3P/CdSe Nanocrystals

  • Following Transformations at Nanoscale - Mechanism of Ion Exchange in CdSe/Cu3P/CdSe NanocrystalsFollowing Transformations at Nanoscale - Mechanism of Ion Exchange in CdSe/Cu3P/CdSe Nanocrystals
  • Following Transformations at Nanoscale - Mechanism of Ion Exchange in CdSe/Cu3P/CdSe Nanocrystals
  • Fig. 1: STEM and TEM characterization of CdSe/Cu3P/CdSe NCs at RT: a) low magnification HRTEM image of NCs in different orientations; b) HAADF-STEM image showing several heterostructures with truncated hexagonal bi-pyramids habit; c) STEM-EDS linear map revealing the distribution of Cd, Se, Cu and P atoms across a single epitaxial-related NC; d) HRTEM planar view of a single NC in [0001] zone axis, revealing translational Moiré. The inset shows 2D-FFT pattern with Moiré reflections and multi-diffraction satellite spots due to the multi-domains heterostructure; e) HRTEM cross-view showing epitaxial growth with axial symmetry; f) corresponding 2D-FFT patterns of CdSe and Cu3P with their {0002} isostructural atomic sketches and general morphology. Figure 1a, c,d are reprinted with permission from ref. [6].
  • Fig. 2: HRTEM images and relevant 2D-FFT of CdSe/Cu3P/CdSe from RT to 450 °C: a) RT analysis showing a typical epitaxial-related NC with separated CdSe and Cu3P domains; b) Intermediate step at 300 °C displaying a loss of low frequency lattice periodicities in Cu3P phase; c) HRTEM at final 450 °C displaying a single NC with a completely changed structure, consistent with fcc Cu2-xSe phase, and not displaying inner interfaces.
  • Fig. 3: EFTEM sequence of zero loss images and Cu, P, Cd and Se elemental maps of CdSe/Cu3P/CdSe NCs, acquired at RT (upper line panels) and 450 °C (lower line panels). The RT analysis shows the elements distribution within several NCs according to their starting epitaxial CdSe/Cu3P/CdSe geometry. The elemental maps at 450 °C are consistent with a Cu-Se phase after loss of Cd and P species. Scale bar: 50 nm.

In situ transmission electron microscopy has been used to study thermally activated ion exchange in nanocrystals (NCs) constituted by one Cu3P domain included between two CdSe domains. Above a thermal threshold Cd and P are taken out from the nanocrystals, while Cu diffuses from the central domain in the external ones substituting Cd. Se follows the inverse path, substituting P in the central domain. Once the reaction is completed, the final NCs are composed by a single Cu2Se domain.

Introduction

One of the most challenging topics of transmission electron microscopy has always been represented by the direct study of transformations occurring on the sample during its observation, the so-called in situ TEM [1]. Nowadays, the last generations of aberration-corrected transmission electron microscopes (TEM) and scanning TEM (STEM), as well as of ancillary devices such as Gatan image filters (GIF) and energy dispersive X-Ray spectrometry (EDS) detectors, allow to reach resolutions never attained before, making then possible to investigate with the finest detail both structure and chemical composition of materials at atomic scale. All that, together with the recent development of novel environmental TEM and in situ TEM sample holders having very limited spatial drift, is opening new scenarios in the direct highly spatially and temporally resolved TEM study of transformations occurring to several compounds as a consequence of an external stimulus [2].

In this direction, colloidal nanocrystals (NCs), due to their small size and reactivity, represent ideal candidates for in situ experiments, as they can be synthesized over a large range of possible morphologies, chemical compositions and crystalline structures [3]. A very promising research field is represented by ion exchange reactions in NCs phases, dispersed in solution, when properly "stimulated" by appropriate conditions. Cation exchange (CE) and anion exchange (AE) reactions lead to total or partial replacement of atom species in one of the corresponding ionic sublattices with new ionic species [4,5].

This kind of reactions could be also performed by an in situ TEM approach, as we show in the following.

Results and Discussion

Room Temperature Characterization
In situ TEM experiments of thermally activated exchange reactions occurring at solid state were performed on heterostructured NCs made of CdSe and Cu3P, triggering both the diffusion of Cu into CdSe regions and of Se into the Cu3P region, with the concomitant sublimation of Cd and P species under high vacuum. These heterostructures are composed by a central domain of hexagonal Cu3P and two lateral domains of hexagonal CdSe, grown in epitaxial relationship with the Cu3P one. The synthesis procedure of as-obtained CdSe/Cu3P/CdSe NCs was reported elsewhere [6].

At room temperature (RT) CdSe/Cu3P/CdSe NCs appeared like truncated hexagonal bipyramids with well-developed peripheral facets, as shown by high-resolution TEM (HRTEM) and STEM images (fig. 1a, b, c). The Z-contrast imaging, performed in high angle annular dark field (HAADF) STEM geometry, was able to discriminate both CdSe and Cu3P domains, whose chemical composition were also spatially resolved through STEM-EDS linear mapping (fig. 1c). The epitaxial growth of CdSe over Cu3P caused the formation of simple Moiré translational pattern, as clearly shown in HRTEM imaging at low and high magnification (fig. 1a, d). The commensurate Moiré fringes could be rationalized considering the interference of CdSe (1120) and Cu3P (3030) lattice sets, with interplanar spacings of 2.14 Å and 2.00 Å, respectively, along a common parallel direction, CdSe (1120) and Cu3P (3030). Two-dimensional fast Fourier transform (2D-FFT) of HRTEM images revealed the superposition of the two structures with multi-diffraction contributions, depicted as satellite spots around the main reflections (inset of fig. 1d). The epitaxial growth of CdSe over Cu3P was displayed through cross-view HRTEM imaging of NCs (fig. 1e) and could be defined by the symmetry laws CdSe (1120) // Cu3P (3030) and CdSe (0002) // Cu3P (3030) for axis and planar orientation, respectively (fig. 1e, f). In particular, hexagonal CdSe and Cu3P are isostructural phases with very similar (0002) bi-dimensional lattices (fig. 1f, lower left panel).

High Temperature Characterization
These NCs, when heated in situ under TEM´s high vacuum, underwent pervasive structural and chemical modifications due to the activation of an ion exchange reaction at solid state. This thermally activated transformation was studied through in situ HRTEM and energy filtered TEM (EFTEM) analysis by heating the NCs system up to 450°C using an in situ TEM heating holder. Such in situ HRTEM characterization started to exhibit an incipient structural modification of Cu3P domains at 300°C, showing a partial loss of the low frequency lattice periodicities (2020 and 1010 spots and their symmetric reflections in the 2D-FFT pattern), whereas CdSe domains remained substantially unchanged (fig. 2a, b). Further temperature increase activated exchange reaction within CdSe/Cu3P/CdSe heterostructures with subsequent disappearance of epitaxial interfaces. At 450°C the crystal structure of NCs was totally modified and the exchange reaction completed. HRTEM structural characterizations and 2D-FFT analyses, performed at high temperature, exhibited d-spacing and crystal vector relationships consistent with only fcc Cu2-xSe phase: every CdSe/Cu3P/CdSe NC was converted into a corresponding single cubic Cu2-xSe NC (fig. 2c). During the thermally activated exchange reaction Cu species diffused from Cu3P into CdSe substituting Cd atoms and at the same time Se species migrated into Cu3P substituting P anions. This process of double inverse diffusion triggered both the crystal structure transformation and the thermal sublimation of Cd and P species under the TEM high vacuum condition. In particular, the crystal direction of anion close-packing was maintained passing from hexagonal to cubic structure, namely from [0001] to [111].

EFTEM analysis of NCs confirmed the chemical transformation involved in the ion exchange reaction at high temperature. RT characterizations exhibited the expected chemical differentiation, due to heterostructured nature of NCs, showing spatially resolved CdSe and Cu3P domains (fig. 3, upper line panels). On the other hand, EFTEM mapping performed at 450°C clearly displayed the final result of exchange reaction, with the migration of Cu and Se species across every heterostructured NC and the sublimation of Cd and P species, which are no longer detected (fig. 3, lower line panel).

Acknowledgment

Prof. Liberato Manna and his team from the Nanochemistry Department of Italian Institute of Technology are gratefully acknowledged for providing all the samples.

References
[1] Ross F.M.: In: Science of Microscopy, ed. P.W. Hawkes, J.C.H. Spence, Springer, 2008, 445-534
[2] Yalcin A.O. et al.: Nano Lett., 14/6, 3661-3667 (2014)
[3] Talapin D.V. et al.: Chem. Rev. 110, 389-458 (2010)
[4] Beberwyck B.J. et al.: J. Phys. Chem. C 117, 19759-19770 (2013)
[5] Gupta S. et al.: Adv. Mater. 25, 6923-6944 (2013)
[6] De Trizio L. et al.: Acs Nano 7, 3997-4005 (2013)

Autors
Prof. Andrea Falqui
Alessandro Genovese
Alberto Casu

King Abdullah University of Science and Technology (KAUST)
Biological and Environmental
Sciences and Engineering Division
Thuwal, Saudi Arabia

Contact

King Abdullah University of Science and Technology (KAUST)
Ibn Al-Haytham
23955-6900 Thuwal
Saudi Arabia
Phone: +96 65 44 7000 60

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