The Effect of Planar Defects on the Optical Properties of Silver Nanostructures

We present a computational, atomistic electrodynamics investigation of the effects of planar defects on the optical properties of silver nanocubes, where the planar defects we considered are different surface orientations, twins, partial dislocations, and full dislocations. We find that for nanocubes smaller than about 3 nm, the optical response is very sensitive to the specific surface structure resulting from the defects. However, the sensitivity, as measured by shifts in the plasmon resonance wavelength, is strongly reduced at larger sizes because of the decreasing importance of surface effects even when the majority of the atomic deformation due to the crystal defects is contained within the interior of the nanocube. Overall, this study suggests that the effects of individual crystalline defects on the optical properties of nanostructures can be safely ignored for nanostructure sizes larger than about 5 nm. ■ INTRODUCTION The optical properties of small metal nanostructures have been investigated in great detail over the past two decades with most of the studies focusing on gold and silver nanostructures. Specifically, these two metals exhibit localized surface plasmon resonance (LSPR), which is a collective oscillation of the conduction electrons when excited by electromagnetic radiation within the visible spectrum. Because of this, they have been utilized for a wide range of applications, including biosensing, single molecule sensing and detection via surface-enhanced Raman scattering (SERS), photothermal ablation treatments for cancer, optical tagging and detection, strain sensing, metamaterials, and many others. In conjunction with these fascinating applications and experiments, computation has played a key role in elucidating the factors that control the LSPR wavelengths and optical properties of the nanostructures. For example, the discrete dipole approximation (DDA) has been one of the most widely used computational techniques to study the optical properties of metal nanostructures. Other popular approaches to calculating the optical properties of metal nanostructures include the finite difference time domain (FDTD) method, volume integral methods, and finite element methods. While these numerical methods have led to many seminal and important conclusions about the optical properties, there exist two main issues that preclude the application of existing numerical methods to the study of crystal defects on the optical properties. First, methods such as DDA and FDTD are both based on the bulk dielectric function, that is, the averaged atomic response of the material, and therefore do not resolve the optical properties at the atomic scale. Second, most numerical techniques work by discretizing a three-dimensional volume into a collection of small subdomains or volumes. Because of this, they are generally unable to explicitly represent the discrete atomic positions and, thus, the exact microstructure of a nanomaterial. For example, in the DDA, the volume is meshed into a set of discrete dipoles, which are constrained to sit on a regular, cubic grid. Because of this, the positions of the dipoles do not correspond to the actual positions occupied by atoms in an ideal face centered cubic (FCC) lattice. Taken one step further, one would experience considerable difficulty in applying these methods to study the optical properties of metal nanostructures that contain planar defects, and as a result, aside from a few studies that considered pinhole-type defects in metal nanoshells, the effect of planar defects on the optical properties of metal nanostructures is largely unknown and unresolved. However, defects are common in metal nanostructures. For example, in solution-phase synthesized pentagonal nanowires or nanorods, the pentagonal nanowire also contains 5-fold twinned symmetry with the twins meeting in the center of the Received: April 26, 2013 Revised: June 10, 2013 Published: June 11, 2013 Article