Capillary forces affect the atomic structure, and can lead to radically different atomic configuration in nanoscale compared to bulk counterpart. In this work, we demonstrate such a phenomenon in ultrathin Au nanowires. These Au nanowires can be fabricated by a simple wet-chemical approach, using oleylamine as a capping agent facilitating the anisotropic crystal growth. Detailed investigation of the atomic structure shows that the close-packed plane normal to the [111]-wire axis undergoes wrinkling leading to the formation of a saddle surface¹. This phenomenon was captured in ab initio simulations (Fig. 1) and was corroborated with aberration-corrected transmission electron microscopic studies (Fig. 2). The reason of such relaxation can be attributed to the anisotropic surface stress of the bounding facets leading to out-of-plane displacement of the atoms. In terms of electronic structure, such atomic scale structural relaxation forces the d-band of the Au nanowire towards higher energy, and opens up a tantalizing possibility of using them as nanoscale sensors², as electronic states near Fermi energy become available for hybridization.

Furthermore, we have generalized this phenomenon for other FCC metal nanowires such as Cu, Ag and Pt. Our recent simulations show that the strain in the FCC nanowire exhibits a systematic variation with the resultant stress along the nanowire orientation (Fig. 3). These results shed light on the atomic structure of FCC nanowires and open up a possibility of experimental investigation of the atomic structure for other FCC nanowires in detail.

References:

1. A. Roy et al., Nano Lett., 14, 4859-4866 (2014)

2. A. Roy et al., J. Phys. Chem. C, 118, 676-682 (2014)

Figures:

FIGURE. 1. a. A [111]-oriented Au NW sculpted from bulk having {110} bounding facets; b,c. Relaxation –induced formation of saddle surfaces in the wire. Although axial <111> projection looks same both in unrelaxed and relaxed state, the side-view i.e. <112> and <110> projections clearly depict the atomic modulation

FIGURE 2. a. Low magnification bright-field TEM image of Au nanowires; b. HRTEM image showing the single crystalline nature of the wires; c. AC-HRTEM image of the wire showing atomic displacements in the nanowire {111} planes; d. same as c, but is a simulated image from relaxed atomic structure under experimental conditions

FIGURE 3. a. Axial atomic displacements with anisotropic surface stress of bounding {110} facet of the metal nanowire; b. schematic showing the resultant surface stress along the nanowire orientation; c. systematic variation of axial atomic displacements with the resultant surface stress along the wire orientation

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EMC Abstracts - https://emc-proceedings.com/abstract/atomic-relaxation-in-ultrathin-fcc-metal-nanowires/