FEM And Near-field Simulations: A Vital Mechanistic Toolfor Studying Silver-based Plasmonic Systems

Ramesh Asapu1, Sammy Verbruggen1,2, Nathalie Claes3, Sara Bals3, Siegfried Denys1 and Silvia Leanerts1

1Department of Bioscience Engineering, DuEL, University of Antwerp, Belgium2Centre for Surface Chemistry and Catalysis, COK, KU Leuven, Belgium3Department of Physics, EMAT, University of Antwerp, Belgium

Silver Plasmonic Systems: What and Where?

Surface Plasmon Resonance:

Collective oscillation of conduction electrons at the dielectric-metal interface of a nanoparticle stimulated by incident light of matching wavelength.

Highest near field enhancement by silver among the plasmonic noble metals like Au, Ag, Pt, Cu etc.

Size tuneable plasmonic properties – FEM vital tool for analysis

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*Darya Radziuk et al. Phys. Chem. Chem. Phys., 2015, 17, 21072

Applications:

Biophotonics

Sensing, Imaging and Therapeutics

SERS

Identification of chemical species

Plasmon enhanced semiconductor photocatalysis – TiO2 systems.

Time domain simulation of Ag NP (~20 nm) in air, λinc = 355 nm

Modeling of plasmonic nanoparticles in COMSOL

COMSOL Wave Optics physics in wavelength domain study.

Solution to Maxwell’s electromagnetic wave equation:

𝛻 ×1

µr𝛻 × E − K0

2(Ɛr−j σ

ωƐ0)E = 0

where E – scattered electric field

K0 - wavenumber in free space

µr - relative permeability of medium

Ɛr – permittivity of medium

PML layers to truncate the domain and avoid internal reflections

Linear polarized plane wave

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Where Qh is total power dissipation density

relPoavx,y,z are the time average power flow of relative fields

Z0_const is the scaling factor

Mie solution to Maxwells’s equations: Implementation in COMSOL

• Mie Solution to Maxwell’s wave equation to calculate the extinction efficiency. (for particles d<< λ)

Absorption cross section Cabs =𝑊𝑎𝑏𝑠

𝐼𝑖1*

Scattering cross section Csca = 𝑊𝑠𝑐𝑎

𝐼𝑖2*

Wabs, Wsca are energy rates absorped and scattered by particle and Ii is energy flux of the incident wave.

Cext= Cabs + Cabs

Qext = 𝐶𝑒𝑥𝑡

𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝑐𝑟𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒3*

*Bohren and Huffman, Absorption and scattering of light by small particles, 1983 Wiley

DOI: 10.1002/9783527618156

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Ag NP

𝐶𝑎𝑏𝑠 = 𝑒𝑤𝑓𝑑. 𝑄ℎ𝐸20

2 ∗ 𝑍0_𝑐𝑜𝑛𝑠𝑡

𝐶𝑠𝑐𝑎 = (𝑛𝑥∗ 𝑒𝑤𝑓𝑑. 𝑟𝑒𝑙𝑃𝑜𝑎𝑣𝑥 +𝑛𝑦 ∗ 𝑒𝑤𝑓𝑑. 𝑟𝑒𝑙𝑃𝑜𝑎𝑣𝑦 + 𝑛𝑧 ∗ 𝑒𝑤𝑓𝑑. 𝑟𝑒𝑙𝑃𝑜𝑎𝑣𝑧)

𝐸202 ∗ 𝑍0_𝑐𝑜𝑛𝑠𝑡

(volume integral over the nanoparticle)

E0

(Surface integral over the nanoparticle)

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Modeling of Ag nanoparticle

Ag silver nanoparticles exhibit high near field enhancement

Prone to oxidation forming a diffuse Ag2O layer effecting the near field enhancement significantly.

Not suitable for applications over long period of time or oxidative conditions.

λInc = 355 nm

E

k5 nm

Ag2O shell 2 nm

λInc = 355 nm

E

k5 nm

a) b)

E-Field enhancement contour of Ag nanoparticle

λInc = 355 nm

E

k5 nm

Ag2O shell 2 nm

λInc = 355 nm

E

k5 nm

a) b)

E-Field enhancement contour of Ag@Ag2O nanoparticle

Silver colloidal nanoparticles stability test in air

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Ultrastable Ag nanoparticles:

Encapsulation of Ag NPs with ultrathin protective polymer shell using LbL method.

4 layers Ag/(PAH/PAA)2

shell 1.4 nm

8 layers Ag/(PAH/PAA)4

Shell 2.4 nm

Bare Ag nanoparticle dia~18nm

Effect of polymer shell on the field enhancement of core-shell nanoparticles.

polymer shell thickness (nm)

0 2 4 6 8 10

Ma

xim

um

en

ha

nce

me

nt fa

cto

r |E

/E0|2

0

100

200

300

400

500

TEM Characterization

Centrifuge to remove excess

polymer

Ag NP

Polyelectrolyte

Ag NPOne cycle

each of

polycationand polyanion= One Bilayer

20 min stirring under dark

Centrifuge Final washing step

Redispersed in Milli-Q water

Validation of models:

Parametric sweep of incident wavelength to generate extincion plots (Mie solution implementation in COMSOL in water and npolymershell = 1.48

Experimental absorption spectra compared with COMSOL model and Mie analytical solution using Bohren and Huffman’s BHCOAT (implemented in MATLAB) for coated nanoparticles. Data from J&C – Jhonson and Christy

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4.34.8

5.4

400.9

386.9 386.7

405.2

391.7 392.1

365

370

375

380

385

390

395

400

405

410

415

0

2

4

6

8

10

12

14

Experimental Mie Analytical J&C Comsol J&C

Surface P

lasmo

n R

eson

ance λ

SPR

max

Red

sh

ift

in S

urf

ace

Pla

smo

n R

eso

on

ance

(n

m)

SPR Red shift SPR-Layer 0 SPR-Layer 4

8 layers Ag/(PAH/PAA)4 2.4 nm shell

4 layers Ag/(PAH/PAA)2 1.4 nm shell

Bare Ag anoparticle 18 nm

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Ultrastable Ag plasmonic nanoarrays for multi-domain applications:

Ag nanoparticle arrays generate hot spots SERS : EF4 ~ 108-1011

Engineering of Nano arrays based on the feedback from E-field simulations. Mesh convergence study for core-shell nanoparticle dimers

Mesh Density Number of Elements Computation time [s] Max point of (Norm. E-field)2

Normal 10374 9 3.13E+05

Fine 16588 11 4.64E+05

Finer 42048 24 4.21E+05

Extra fine 135833 85 3.50E+05

Extremely fine 647861 609 3.45E+05

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Ag plasmon enhanced TiO2 gas phase photocatalysis

Application of silver nanoparticles for long-term stable plasmonenhanced gas phase photocatalysis.

Acetaldehyde as a model pollutant in gas phase photocatalysis

FEM numerical simulations to corroborate experimental evidence to identify the major mechanism responsible for plasmonic enhancement.

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18.6

25.2

21.0

17.0

19.821.3

18.216.8

22.0

0

5

10

15

20

25

30

P25 P25_Ag P25_Ag_L4

% A

ceta

ldeh

yde

deg

rad

atio

n

Long term stability study of Ag-TiO2 photocatalytic system using 4 layered Ag core-shell nanoparticles

t=0 t=4 weeks t=16 weeks

*Ramesh Asapu et al. Applied Catalysis B: Environmental, 200 (2017), 31-38

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Ag plasmon enhanced TiO2 photocatalysis

Ag@polymer core@shell nanoparticles to study near-field / charge transfer

Insulating polymer spacer layer rules out charge transfer

So how distant the near field enhancement is helpful!

FEM simulations provide an estimation feedback for experimental synthesis

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TiO2

Substrate

EF d

istr

ibu

tio

n m

aps:

log

(E2/E

02)

TiO2

TiO2

TiO2

TiO2

TiO2

TiO2Ag NP

Ag core-shell NP

1.4 nm

Ag core-shell NP

4 nm

Influence of spacer layer between Ag plasmon and TiO2 nanoparticles on the enhancement

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Conclusion:

FEM simulations can provide crucial insights: from synthesis, design and application perspective

Study the effect of medium and design of nanoparticle plasmonic system for wide domain of applications

Vital mechanistic tool : plasmon enhanced photocatalysis and hotspot applications

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Thanks for your attention