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Synthesis, structure and properties of CuAg Nanoparticles

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Core-shell structures are particularly interesting due to the property tuning via the control of several structural and chemical parameters like the core size, the shell thickness, the size ratio between the core and the shell as well as the chemical arrangement and interface structure. Surface Plasmon Resonance (SPR) and magnetic properties of such nanoparticles can then be adjusted to desired values through the modification and control of these parameters.
The synthesis route plays an important role for mastering the nanoparticle properties. Here we use ultra-high vacuum technique to obtain core-shell bimetallic nanoparticles on a substrate by sequential deposition. Either thermal evaporation and pulsed laser deposition are used for producing the metallic vapors that condense on the substrate. The Cu@Ag (core@shell) system has been chosen for this study to investigate the growth mechanisms and the chemical configurations formed by two immiscible metals, with different atom sizes and mean surface energies. Our results show that the deposition of copper vapor on an amorphous carbon film leads to copper nanoparticles with a structural evolution depending on the size (icosahedron -> decahedron -> cube-octaedron) as already observed for other noble metals. The next step consists in the deposition of silver vapor onto the Cu nanoparticles. This step results in a strong modification of the size distributions of the nanoparticles. A silver-driven coalescence between the copper nanoparticle occurs that results in the formation of core-shell nanoparticles, as illustrated here after :

image 1 CuAg
Figure : chemical map of Cu@Ag core-shell nanoparticles (red = copper ; green = silver) obtained by energy filtered imaging.

The thermal stability of the Cu@Ag core-shell nanoparticles has been investigated by in-situ TEM. The main result is that the core-shell structure is stable up to 600°C provided the surface diffusion of metallic species on the carbon film is blocked. This has been achieved with an amorphous carbon film deposited on the nanoparticles. On the contrary, without any carbon capping, the temperature-enhanced surface diffusion leads to Ostwald ripening and coalescence between the nanoparticles. The final chemical configuration is a so-called Janus nanoparticle, side-segregated arrangement, illustrated on the next figure :

image 2 CuAgFigure : chemical map of Cu@Ag core-shell nanoparticles after annealing at 550°C for 1h (red = copper ; green = silver) obtained by energy filtered imaging.

The interface configuration is also of great interest for understanding the stability of the different chemical configurations. The Cu@Ag core-shell nanoparticles structure has been investigated by ultra-high resolution scanning transmission electron microscope equipped with a spherical aberration corrector of the condenser lens. The high resolution images clearly evidence the cube-on-cube and heterotwin epitaxial relationships between the core and the shell, identical to the one found in eutectic CuAg alloys between lamellae. Moreover, because of the lattice misfit of 12% between Cu and Ag, an array of interface dislocation is observed all around the particle, with a periodicity corresponding roughly to the theoretical one.

image 4 CuAg
Figure : a) HAADF-STEM image. b) same image after Fourier filtering, highlighting the interface dislocation network between the copper core (green) and the silver shell (red).

Because the bimetallic core-shell nanoparticles show interesting surface plasmon resonance, we plan to characterize locally the plasmon resonance of a single nanoparticle inside a TEM, using a gun monochromator allowing an energy resolution of few tenth of eV. It will be very interesting to compare the resonance of several particles having different chemical configurations (core-shell ; Janus, side-segregated, different size ratio between core and shell...) and compare with the global results from optical UV-Visible spectroscopy.

Collaboration :
Raul Arenal (LEM-ONERA, France) ;
Ziyou Li, Zhiwei Wang et Richard Palmer (Nano-Physics Research Laboratory, University of Birmingham, UK) ;
Jun Yuan (Physics Department, University of York, UK)

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