Defects twinning

Fig. 22. ETEM at 180°C in N2, illustrating the stability of gold nanorods, for nanoelectronics and catalysis applications. Gold atomic layers and surface atomic structures are visible. Surface of gold nanorod at room temperature showing twin defect lamellae on the atomic scale. They indicate interaction of the surfactant with the (110) surface forming twins to accommodate the shape misfit between the two.

Figure 10.14 (a) TEM and (b) HRTEM images ofYBa2Cu307 on the [001] projection, showing twin defects. The inset of (a) is a schematic model of twin defects. In (b), the a axis across the twin planes is marked by the white lines. 

In a crystal containing twin defects, the crystal lattices continue across the twin boundaries without a break. Another similar defect, the antiphase defect, is formed by a shift of the crystal by half a unit cell along the antiphase boundary. This defect can also contribute to strong image contrast as shown in Figure 10.3b. 

The absence of any preferred zone combination in the electron diffraction patterns suggests that the side faces of the nanorods are either not well-developed or consist of two forms, e.g. 100 and 110 with approximately equivalent surface area. HRTEM images of individual gold nanorods (Figure 9.5) show stripe patterns characteristic of the superposition of two diffraction patterns, i.e. a twinned defect structure, consistent with the SAED data. 

It was also possible, by HRTEM measurements, to observe defects in the crystalline lattice of the Pt(0) NPs prepared in BMI.BF4 (Fig. 6.7). Note that twin defects are typical for small particles. 

Defect and Twinned Plane Since many metals have fee structures, there is no strong intrinsic driving force for metals or alloys to grow into one-dimensional (ID) or two-dimensional (2D) nanocrystals. However, these two classes of highly anisotropic shapes can be obtained in cases when the cubic symmetry is broken down, which is achievable by introducing twin defects in the seed nanoparticles. For... 

Figure 6. Etch pits of dislocations with increasing thickness of molybdenum deposit on the (001) plane of molybdenum substrate (a,b) c- the twin defect formation. x200 a - substrate, b - molybdenum deposit (thickness - 0.05mm) c - molybdenum deposit (thickness - 0.1mm).

The total excess free energy can further decrease by introducing crystal defects. Twinning defects, those in (111) plane for fee metals in particular, are the most common and widely observed type in the shape control of metal nanostructures [38, 39]. The introduction of twin planes reduces the symmetry and alters the... 

As with the 1-D nanostructures discussed in Section 10.5, the formation of 2-D Pt nanostructures relies upon either defects in seed crystals or using templates. The most commonly encountered defects with platinum metal are those due to stacking faults, such as twin defects in (111) planes. Lipids and micelles at the interface are the types of soft template most useful for the generation of 2-D nanostructures. Some TEM images of representative, recently created 2-D Pt nanostructures are shown in Figure 10.10 these structures include planar multipods (bipods and tripods), triangular plates and dendritic sheets. 

Roles of twin defects in the formation of platinum multipod nanocrystals. Journal of Physical Chemistry C, 111, 14312. 

Figure 11.5 Schematic of conventional shapes of face-centered cubic (fee) metal nanostructures. The shapes in the top row are single crystals, in the second row are particles with twin defects or stacking faults, and in the...

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