(185h) on the Structure of Pentagonal Cross-Section Nanowires [Invited]

One of the most interesting features of the silver nanowires produced by the polyol method in presence of PVP is their remarkable structure. Their cap resembles a decahedron and there is experimental evidence of a pentagonal cross-section all along the long axis of the nanowire (1, 5), with a contrast quite similar to this of the multiple twinned particles (MTP). These features have been also reported in nanowires with pentagonal cross-section of gold and copper. Based on these observations, it has been proposed that they evolved from a multi twin decahedral nanoparticle growing in the [110] direction with the capping agent assisting to direct the structure by stabilizing more effectively the new formed {100} facets than the {111} facets.

Recently, gold, copper and silver, copper nanowires have been synthesized. All of them present many interesting features such as their striking five-fold twinned structure with pentagonal cross-sections. Even when the growth mechanism needs to be completely elucidated, it has been proposed that they evolved from multi twin decahedral nanoparticles. However, up to today there is not a complete explanation of all the presented features in the electron diffraction generated along the long axis of this type of nanowires. Herein, we present a comprehensive study on the structure of these multi twin decahedral based silver nanowires (Dh-NWs).

Pentagonal arrangement in MTPs is quite known. MTP nanoparticles of transition metals with face-centered cubic (FCC) lattice (7-9) and some related materials such a as carbon (9) and silicon (10) have been reported. Based on these studies, the basic structure of a decahedral particle can be described as the junction of five tetrahedron single crystals with twin-related adjoining faces. The theoretical angle between two (111) planes is ~70.5°, so by joining 5 tetrahedrons, which are bounded by {111} facets, a gap of ~7.5° is generated. Thus, the space can not be filled by just joining five tetrahedrons and some form of internal strain is necessary, giving place to dislocations and other structure defects (7-9). These defects are also observed in the transmission electron microscope (TEM) cross-section images of the mentioned silver nanowires (1).

The structure of these Dh-nanowires has been described as five triangular prisms joined in such a way that they show the {100} planes on their sides and are capped by {111} planes, growing along the [110] direction (1, 4-6). However, up to now it is not clear the actual physical mechanism that allows the growth of such structure having a decahedral cap and be energetically stable. It is clear that a comprehensive understanding of the structure is required to elucidate the growth mechanism.

In this work, we present and discuss the structure of this type of silver nanowires through the analysis of high resolution TEM (HRTEM) images and their selected area electron diffraction (SAED) patterns. We found that both the HRTEM images and the SAED patterns can be easily interpreted in the basis of a multi-twin decahedron. In particular, we found that the observed contrast of HRTEM images can be explained in terms of Moire fringes generated from the overlapping of the silver FCC unit cell planes arranged in the five tetrahedra that compose the decahedron. Finally, we proposed that the structure of these nanowires is better interpreted as a chain of decahedra joined along the vertex, which is parallel to the 5-fold symmetry.

Silver nanowires were synthesized by the polyol reduction of silver nitrate (AgNO3) in presence of PVP. Scanning electron microscopy of the nanowires was conducted using the scanning electron microscopes (SEM) Hitachi 4500F operated at 15 kV, and a Cs-Corrected SEM JEOL-JSM7700F microscope operated at 2kV and 30kV. This last SEM microscope allows us to observe the sample in different modes, i.e. SEM, TEM and scanning TEM (STEM) with a resolution of 0.6 nm. Transmission electron microscope was conducted in a HRTEM JEOL 2010F microscope equipped with Schottky-type field emission gun, ultra-high resolution pole piece (Cs = 0.5 mm), and a scanning transmission electron microscope (STEM) unit with high angle annular dark field (HAADF) detector operating at 200 kV, and a Cs-corrected FEI-TECNAI microscope operated at 200kV. For digital image processing the Digital Micrograph (GATAN) software was used. To simulate the electron diffraction patterns, the SimulaTEM software (11) was employed.

We show that their high resolution transmission electron images (HRTEM) can be interpreted as a Moire pattern contrast based on a multi twin decahedron and that their selected area electron diffraction (SAED) patterns can be also completely generated through the same multi twin decahedron basis. We proposed that the structure of these nanowires can be interpreted as a chain of decahedra along the 5-fold symmetry axis and joined in the vertex.

Acknowledgements The authors gratefully thank to Domingo I. Garcia and Samuel Tehuacanero for the technical assistance during this work and R. Castillo for his useful comments. JLE and ACB acknowledge the support received from CONACyT-Mexico.

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