Understanding the formation of multiply twinned structure in decahedral intermetallic nanoparticles

Abstract

The structure of monometallic decahedral multiply twinned nanoparticles (MTPs) has been extensively studied, whereas less is known about intermetallic MTPs, especially the mechanism of formation of multiply twinned structures, which remains to be understood. Here, by using aberration-corrected scanning transmission electron microscopy, a detailed structural study of AuCu decahedral intermetallic MTPs is presented. Surface segregation has been revealed on the atomic level and the multiply twinned structure was studied systematically. Significantly different from Au and Cu, the intermetallic AuCu MTP adopts a solid-angle deficiency of -13.35°, which represents an overlap instead of a gap (+7.35° gap for Au and Cu). By analysing and summarizing the differences and similarities among AuCu and other existing monometallic/intermetallic MTPs, the formation mechanism has been investigated from both energetic and geometric perspectives. Finally, a general framework for decahedral MTPs has been proposed and unknown MTPs could be predicted on this basis.

Keywords: aberration-corrected scanning transmission electron microscopy; disclinations; intermetallic nanoparticles; multiple twinning.

Figure 1

Morphology and elemental distribution of intermetallic AuCu NPs. (a) Low-magnification TEM of AuCu intermetallic NPs with a 6 nm diameter. Two MTPs are indicated by red circles. (b) XRD and (c) STEM-EDS mapping of intermetallic AuCu NPs.

Figure 2

AuCu intermetallic MTPs and surface Au-enriched shell. (a) Experimental atomic resolution HAADF image of an AuCu MTP and its FFT in (b). Four sets of points are marked with different colors separately and some less obvious points are marked with broken circles. (c) Intensity profiles across the particles measured along the direction of the red arrow from the yellow box shown in (a). (d) Theoretical model of AuCu MTP without a surface Au-enriched shell, its simulated HAADF image in (e) and corresponding intensity profile in (f). (g) Theoretical model of AuCu MTP with a surface Au-enriched shell and its simulated HAADF image in (h) and corresponding intensity profile in (i).

Figure 3

Space imperfection in different decahedral MTPs. (a) A unit cell of Au single crystal with f.c.c. structure. A b.c.t. repeating unit can be extracted from the f.c.c. structure. (b) Structural projection with a blue triangle which corresponds with a segment of decahedral MTPs. (c) Diagram of Au decahedral MTP comprising a gap of 7.35°. The blue triangles with 70.53° angles of the top three images have a one to one correspondence. The main expansion direction [110] has been marked with red broken lines. (d) A unit cell of an AuCu single crystal with an f.c.t. structure. A b.c.t. repeating unit can be extracted from the f.c.t. structure. (e) Structural projection with a blue triangle that corresponds with a segment of intermetallic decahedral MTPs. (f) Diagram of AuCu decahedral MTP comprising an overlap of 13.35°. The blue triangles with 74.67° angles of the bottom three images have a one to one correspondence. The main expansion direction [001] has been marked with blue broken lines.

Figure 4

Eccentricity and energy barrier of disclination. (a) Diagram of small particles with different eccentricity which can be evaluated by parameter β ranges from −1 to 1. (b) Curves of the disclination energy barrier with different eccentricity of different MTPs.

Figure 5

Strain distribution and analysis of lattice distortions. (a) Two-dimensional lattice strain map of the AuCu intermetallic decahedral MTP. (b) Strain of decahedral MTPs plotted as a function of the disclination rotation angle for both experimental and predicted MTPs. The two lines are the α–ω relationship when K = 1.02 (red) and K = 1 (blue). (c) Histogram of the d _A and d _B contribution to the expansion for both experimental and predicted MTPs.