Copper sulfide (Cu_2-xS) nanocrystals (NCs) gained significant attention during the past decade as functional materials for photovoltaics and photothermal applications owing to their high electrical conductivity, low cost and environmental friendliness. Phase transformations of Cu_2-xS NCs upon chemical reactions have been extensively studied [1-2], while their thermal stability, crucial in view of their integration into devices, has not been carefully addressed so far. The main aim of the present study was to monitor thermally-induced transformations within colloidal Cu_2-xS NCs of different aspect ratios, simple and decorated by noble metal NCs. The structural and compositional evolution undergone by NCs was carefully investigated at the nanoscale by transmission electron microscopy (TEM)-related techniques.

The Cu_2-xS NCs were obtained via cation-exchange (Cd²⁺->Cu⁺) on colloidal CdS NCs synthesized according to previous reports [3-5]. This study focuses on two main types of NCs: nanorods (NRs, 20±5 nm diameter, 45±5 nm width) and nanowires (NWs, 20±2 nm width and several μm long). In-situ heating-up experiments were performed by a JEOL heating holder (maximum temperature 800°C) within an image-C_S-corrected JEOL JEM-2200FS TEM. The ongoing processes were monitored via high-resolution TEM (HRTEM) and high-angle annular dark field scanning TEM (HAADF-STEM). The evolution of the chemical composition was followed ex-situ via energy-dispersive X-ray spectroscopy (EDS), carried out in STEM mode. Ex-situ heating experiments were carried out within an ultra-high vacuum furnace.

The set of experiments performed evidence the reproducible and homogeneous formation of crystalline copper domains on both NC systems in the temperature range 350°C-450°C. While in the case of NRs a single domain is formed, NWs exhibit Cu domains at a rather regular spacing along their length. As the initial composition and structure (Cu₂S, high chalcocite) are kept in the remaining region of the NCs, the process seems to be induced by simultaneous oxidation and partial sublimation of sulfur in the NCs and reduction and outward diffusion of excess Cu, probably as an effect of thermal annealing of structural defects (stacking faults, twins) deriving from the CdS synthesis process. This process occurs only in case of relatively high heating rates (~25°C/min). In case of noble metal deposition (Au, Pt) on these Cu₂S NCs, either by sputter-coating or by colloidal growth, the fast diffusion of the noble metal atoms on the NC surface (starting at about 100°C for Au) induces a regular arrangement of domains which acts as a template for the following Cu out-diffusion, making the deriving hetero-structure more robust. The observed reproducible production of metal-semiconductor heterostructures by a simple post-synthesis heating treatment of colloidal Cu₂S NCs opens a path for the production of low cost materials for optoelectronics, as it can simply be extended to a huge range of material combination, for instance achievable by cation-exchange following the heating treatment, which will affect only the Cu₂S segments of the obtained hybrid nanostructures and give rise to application-relevant structures.

[1] Y. Xie, A. Riedinger, M. Prato, A. Casu, A. Genovese, P. Guardia, S. Sottini, C. Sangregorio, K. Miszta, S. Ghosh, T. Pellegrino, L. Manna, J. Am. Chem. Soc. 135, 17630 (2013)

[2] V. Lesnyak, R. Brescia, G. C. Messina, L. Manna, J. Am. Chem. Soc. 137, 9315 (2015)

[3] K. Miszta, G. Gariano, R. Brescia, S. Marras, F. De Donato, S. Ghosh, L. De Trizio, L. Manna, J. Am. Chem. Soc. 137, 12195 (2015)

[4] J. Puthussery, T. H. Kosel, M. Kuno, Small 5, 1112 (2009).

[5] S. Naskar, A. Schlosser, J. F. Miethe, F. Steinbach, A. Feldhoff, N. C. Bigall, Chem. Mater. 27, 3159 (2015).

Acknowledgements: The authors acknowledge funding from the European Union under grant agreement n. 614897 (ERC Grant TRANS-NANO).

Figures:

HAADF-STEM images of Cu2S NWs heated up to 400°C: (a) as-synthesised NWs, (b) Au-sputter-coated NWs and (c) NWs after colloidal Au growth. In the insets, images of the corresponding system at room temperature, before heating up.

(a) HAADF-STEM image of a Cu2S NW heated up to 400°C and corresponding EDS quantitative maps (background subtracted, deconvoluted peaks) for Cu and S: the domain forming along the NW upon heating up consists of metallic Cu. (b) HAADF-STEM image of a Au-sputter coated Cu2S NW after heating up to 400°C, with corresponding EDS quantitative maps for Cu and Au: the metallic domains consist of Au-Cu bimetallic alloy nanoparticles.

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