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

X. Ben, P. Cao and H.S. Park
Journal of Physical Chemistry C 2013; 117:13738-13746.

Abstract

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, due to 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.

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Strain Engineering Enhancement of Surface Plasmon Polariton Propagation Lengths for Gold Nanowires

X. Ben and H.S. Park
Applied Physics Letters 2013; 102:041909.

Abstract

We present a new perspective, that of elastic strain engineering, to reducing the intrinsic losses in a metal for subwavelength optical signal processing. By using a simple, analytical waveguide model, we demonstrate that application of uniaxial tensile strains below the yield strain of gold nanowires results in substantial increases of more than 70% in the surface plasmon polariton propagation lengths at optical frequencies. The enhancement is primarily due to a reduction in the core electron density, and is found to be size-independent for a wide range of nanowire diameters, while exhibiting a linear dependence on the applied tensile strain.

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Size-Dependent Validity Bounds on the Universal Plasmon Ruler for Metal Nanostructure Dimers

X. Ben and H.S. Park
Journal of Physical Chemistry C 2012; 116:18944-18951.

Abstract

We study the validity of the recently proposed universal plasmon ruler in the present work using a combination of numerical techniques based on the finite difference time domain (FDTD) method, and semi-analytical theories based on the coupled dipole approximation. By incorporating nonlocal effects for closely spaced two-dimensional gold nanocylinder dimers, we find using both the FDTD and semi-analytical approaches that the universal plasmon ruler of Jain et al. is not applicable for gold nanocylinder dimers with diameters smaller than about 20 nm. The nonlocal effects are also found to strongly reduce the electric field enhancements at very small gap distances. Taken together with previous results, we are able to establish the valid size range for the universal plasmon ruler of gold: 20 nm diameter metal nanostructure dimers at the smaller end, and 70 nm diameter metal nanostructure dimers at the larger end.

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Size-Dependence of the Plasmon Ruler for Two-Dimensional Metal Nanosphere Arrays

X. Ben and H.S. Park
Journal of Physical Chemistry C 2011; 115:15915-15926.

Abstract

We have utilized the discrete dipole approximation to study the localized surface plasmon resonance in infinite, periodic two-dimensional array of gold nanospheres with the nanospheres arranged according to the {100} face of an FCC crystal. Specifically, we have performed a systematic study of the sensitivity of both the plasmon resonance wavelength shift and extinction properties considering nanosphere diameters ranging from 20-100 nm, and for nanosphere gap distances ranging from 0.5 to 6 times the nanosphere diameter. In doing so, we find that the same universal decay length of the plasmon resonance wavelength shift of about 0.2 units of the nanosphere size that was previously found by Jain \emph{et al.}~\cite{jainNL2007} for nanoparticle dimers is also operant for two-dimensional arrays. However, we also find that the universality of the plasmon ruler is only valid for arrays with nanospheres smaller than a critical nanosphere diameter of about 70 nm, while for larger nanosphere diameters a decrease in the extinction efficiency as the gap distance decreases and a reduction in the decay constant are observed. Both of these size-dependent optical responses are qualitatively interpreted using a semi-analytical coupled dipole approximation that accounts for structural retardation due to the geometric arrangement of the nanospheres, as well as single sphere retardation due to both dynamic depolarization and radiative damping effects. Using the semi-analytical theory we find that the size-dependence is primarily due to the effects of dynamic depolarization and structural retardation, which reduces the coupling strength, changes the extinction efficiency trend, and also reduces the decay constant of the plasmon ruler equation for larger diameter nanospheres; similar results were found for infinite, 2D arrays of nanospheres in hexagonal and simple cubic arrangements. Finally, the semi-analytical theory is utilized to predict a size-dependence of the plasmon ruler for dimers starting at the same critical diameter of 70 nm. However, we find that the size effect is weaker for dimers than for the array case due to the significant reduction in structural retardation for dimers as compared to the array case.

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Strain Effects on the SERS Enhancements for Spherical Silver Nanoparticles

X. Qian and H.S. Park
Nanotechnology 2010; 21:365704

Abstract

We demonstrate in the present work through utilization of classical Mie scattering theory in conjunction with a radiation damping and dynamic depolarization corrected electrostatic approximation the significant effect that mechanical strain has on the optical properties of spherical silver nanoparticles. Through appropriate modifications of the bulk dielectric functions, we find that the application of tensile strain generates significant enhancements in the local electric field for the silver nanoparticles, leading to large SERS enhancements of more than 300% as compared to bulk, unstrained nanoparticles when 5% tensile strain is applied. While the strain-induced SERS enhancements are found to be strongest for nanoparticle diameters where radiation damping effects are minimized, we find that the surface plasmon resonance wavelengths are relatively unchanged by mechanical strain, and that the various measures of the far field optical efficiencies (absorption, scattering, extinction) can be enhanced by up to 150% through the application of tensile strain. The present findings indicate the opportunity to actively engineer and enhance the optical particles of silver nanoparticles through the application of mechanical deformation.

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Surface Stress Driven Lattice Contraction Effects on the Extinction Spectra of Ultrasmall Silver Nanowires

H.S. Park and X. Qian
Journal of Physical Chemistry C 2010; 114:8741-8748

Abstract

We utilize numerical simulations based on the discrete dipole approximation to study the effects of surface stress-driven lattice contraction on the extinction spectra of silver nanowires with a square cross section of length 2 nm. The novel finding of the present work is the determination that the blue shift that is induced in the silver nanowires due to surface-stress-driven lattice contraction increases with an increase in the nanowire aspect ratio; the blueshift in the longitudinal plasmon resonance wavelength reaches 20 nm in air and 30 nm in water when the nanowire aspect ratio increases to six. Furthermore, we have delineated the lattice contraction effects on the relative contributions of the free (conduction) electrons and the ionic core (bound) electrons to the observed blue shift; specifically, due to the increasingly free electron optical response of the nanowires with increasing aspect ratio, the blueshift due to the contraction-driven increase in the free electron density is found to dominate the redshift due to the increase in the core electron density for larger nanowire aspect ratios. The results collectively indicate that surface stress-driven lattice contraction plays an important role in blue shifting the longitudinal plasmon resonance wavelength for ultrasmall silver nanowires.

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The Influence of Mechanical Strain on the Optical Properties of Spherical Gold Nanoparticles

X. Qian and H.S. Park
Journal of the Mechanics and Physics of Solids 2010; 58:330-345

Abstract

We utilize classical Mie scattering theory to investigate the effects of tensile and compressive mechanical strain on both the far field (absorption, scattering and extinction efficiences) and near field (surface enhanced raman scattering) optical properties of spherical gold nanoparticles with diameters ranging from 10 to 100 nm. By accounting for the strain effects on both the ionic core (bound) and conduction (free) electrons through appropriate modifications of the bulk dielectric functions, we find that gold nanoparticles are relatively sensitive to the effects of mechanical strain due to the fact that the plasmon resonance wavelength for spherical gold particles, which occurs around lambda=520 nm, is nearly coincident with the interband transitions of the core electrons. Specifically, we find that tensile strain leads to significant enhancements ranging from 60-120% in the far field optical efficiencies, while compressive strain leads to similar decreases, and that the plasmon resonance wavelength can be red or blueshifted up to 100 nm due to the applied strain. Finally, we find that tensile strain also strongly enhances the local electric (E)-field at the surface of the nanoparticles, which is of considerable interest for surface-enhanced Raman scattering applications; 5% tensile strain is found to enhance the |E|^{4} intensity by 63%. The present results demonstrate the potential of mechanical strain, and specifically that of tensile mechanical strain in enhancing and tailoring the optical properties of gold nanoparticles.

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