Refined effective-medium model for the optical properties of nanoparticles coated with anisotropic molecules

Tang, Chhayly; Auguié, Baptiste; Ru, Eric C. Le

doi:10.1103/physrevb.103.085436

Cited by 10 publications

(12 citation statements)

References 59 publications

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“…This can be simply explored within an isotropic core–shell model. The complex wavelength-dependent effective dielectric function of the adsorbate shell can be expressed in terms of the polarizability of the molecules α d as where L m = (ε m + 2)/3 is the local field correction factor , and c d is the adsorbate concentration in the shell. For a coverage ρ (in molecule per unit area) and a small shell thickness L s , c d ≈ ρ/ L s .…”

Section: Resultsmentioning

confidence: 99%

“…One of the difficulties is to relate the properties (e.g., dielectric function) of this macroscopic effective medium to the microscopic properties (e.g., polarizability tensor) of the adsorbate . Often, a simple empirical Lorentz oscillator is assumed but it does not account for any dependence on molecular orientation or coverage, which have been shown to be important. ,, Very recently, we have developed an accurate model for the anisotropic dielectric tensor of adsorbed molecules treated as an effective layer, which we will refer to as the effective layer model (ELM) . This new approach accounts for both molecular concentration and orientation and was validated by comparing it to a microscopic model in the special case of spherical NPs, where extensions of Mie theory can be used to solve Maxwell’s equations in the presence of point dipoles representing anisotropic molecules …”

Section: Introductionmentioning

confidence: 99%

“…First, does molecular orientation affect the adsorbate-induced shift in plasmon resonance? For this, we first follow ref and use anisotropic Mie theory to understand orientation effects on spherical NPs. As plasmon resonances also exhibit anisotropic properties in nonspherical particles, we then extend this study to spheroidal NPs to investigate the interplay between molecular and plasmonic anisotropy.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Molecular Position and Orientation on Adsorbate-Induced Shifts of Plasmon Resonances

Tang

Auguié

2022

J. Phys. Chem. C

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Localized surface plasmon resonances (LSPRs) of metallic nanoparticles are affected by their surroundings and in particular by the presence of adsorbed molecules on their surface. This effect is central to their application in LSPR sensing. We here investigate how the adsorbed molecule orientation on the surface and position, notably with respect to an electromagnetic hot spot, affect the amplitude of the resonance shift, and therefore the LSPR sensing sensitivity. We use a recently developed effective anisotropic dielectric function describing a homogeneous shell of adsorbed molecules combined with anisotropic Mie theory calculations for core–shell spherical systems or finite-element modeling for nonspherical or partial shell configurations. We show that the induced plasmon resonance shift per molecule is strongly correlated with the near-field enhancement experienced by the adsorbed molecules. Molecules with their main optical axis perpendicular to the surface and those located at hot spots can therefore induce a much larger resonance shift, by at least 1 order of magnitude. This work suggests that large improvements in LSPR sensing sensitivity could be achieved with new schemes including targeted adsorption at hot spots with carefully engineered molecular orientation.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Effect of Molecular Position and Orientation on Adsorbate-Induced Shifts of Plasmon Resonances

Tang

Auguié

2022

J. Phys. Chem. C

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“…The new model will then be compared to the solutions from the GCDM of the same problem, dye-molecules surrounding a metallic nanoparticle. The validity of this improved effective medium model was successfully proven in our recent publication [47]. The main advantage of this refined EMM is that its simulation times are less than five minutes to compute the far-field properties of the core-shell system for three different orientations of the dye molecules with dye's surface coverage of (0.01, 0.2 : 0.2 : 1.2 nm 2 ) on a 14 nm silver sphere.…”

Section: Goals and Outlinementioning

confidence: 90%

Computational Modelling for the Optical Properties of Dye Molecules Adsorbed onto Metallic Nanoparticles

Tang¹

2021

Preprint

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The study of light scattering by particles has become fundamental and applied interests in the fields of chemistry, biology, and most importantly in physics. In this context, this thesis focuses on understanding the optical properties of dye layers adsorbed onto metallic nanoparticles (NP), which is essential for interpreting the results of plasmon-dye coupling experiments. To model such a system, Mie theory is often used to solve for the exact solution to Maxwell’s equations for spherical homogeneous and isotropic coated NP. The effects of the NP’s plasmon resonances on the optical properties of the adsorbed dye layer have been predicted using an effective medium model, where the dye-layer is treated as an isotropic layer with an effective dielectric function accounting for the dye resonance. However, this isotropic shell model is inadequate as it cannot account for the dye surface concentration and the anisotropy of the optical response of the dye layer. In this thesis, we introduce anisotropic effects within Mie theory and develop microscopic models to define effective dielectric functions which explicitly include the dye-concentration effect in the shell model. Combining anisotropic Mie theory with a concentration-dependent effective shell model allows us to form new theoretical tools to model the optical properties of adsorbed dye layers on metallic NPs of spherical shape. With this new refined effective medium model, we are then able to study shell models for elongated particles beyond the quasi-static approximation. This is implemented using the finite element method (FEM) to numerically solve Maxwell’s equations. The FEM implementation is then used to investigate how the NP’s plasmon resonance can be affected by the dye’s orientation and location on the NP’s surface. We show that the orientation and location of the dye molecules on the NP determine how strongly the plasmon resonance is shifted. The results of this work will improve our ability to accurately model the optical properties of anisotropic molecules adsorbed on metallic NPs. This is important in a number of applications including the development of localised surface plasmon resonance (LSPR) sensing and the design of plasmonic devices.

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“…In other cases, orientation-averaged crosssections are calculated in the subroutine calcOAprops, which uses the common-origin collective T -matrix T (Eqs. 57,60,63). Note that these calculations based on collective T are much faster than those based on particle-centred T -matrices T (i,j) .…”

Section: Orientation Averaged Cross-sectionsmentioning

confidence: 99%

Multiple scattering of light in nanoparticle assemblies: user guide for the TERMS program

Schebarchov,

Fazel-Najafabadi,

et al. 2021

Preprint

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We introduce terms, an open-source Fortran program to simulate near-field and far-field optical properties of clusters of particles. The program solves rigorously the Maxwell equations via the superposition T -matrix method, where incident and scattered fields are decomposed into series of vector spherical waves.terms implements several algorithms to solve the coupled system of multiple scattering equations that describes the electromagnetic interaction between neighbouring scatterers. From this formal solution, the program can compute a number of physically-relevant optical properties, such as far-field cross-sections for extinction, absorption, scattering and their corresponding circular dichroism, as well as local field intensities and degree of optical chirality. By describing the incident and scattered fields in a basis of spherical waves the T -matrix framework lends itself to analytical formulas for orientation-averaged quantities, corresponding to systems of particles in random orientation; terms offers such computations for both far-field and nearfield quantities of interest. This user guide introduces the program, summarises the relevant theory, and is supplemented by a comprehensive suite of stand-alone examples in the website accompanying the code.

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Refined effective-medium model for the optical properties of nanoparticles coated with anisotropic molecules

Cited by 10 publications

References 59 publications

Effect of Molecular Position and Orientation on Adsorbate-Induced Shifts of Plasmon Resonances

Effect of Molecular Position and Orientation on Adsorbate-Induced Shifts of Plasmon Resonances

Computational Modelling for the Optical Properties of Dye Molecules Adsorbed onto Metallic Nanoparticles

Multiple scattering of light in nanoparticle assemblies: user guide for the TERMS program

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