Faculty of Sciences Thickness-dependent refractive index in - - PowerPoint PPT Presentation

Faculty of Sciences

18th annual workshop of the IEEE Benelux chapter - Mons 2015

• C. Guyot and M. Voué

Physics of Materials and Optics

corentin.guyot@umons.ac.be

Thickness-dependent refractive index in plasmonic nanocomposites

Université de Mons

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Content

• Introduction
• Materials & methods
• Results & discussion

– Topography of the nanocomposite films (AFM) – Optical properties (SE) – Multivariate analysis – SVM classification

• Conclusions
• C. Guyot | Physics of Materials and Optics Unit

Université de Mons

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• C. Guyot | Physics of Materials and Optics Unit

Plasmonic nanocomposites Dielectric matrix Metal NPs Polyvinyl alcohol (PVA) Silver NPs “In situ synthesis” : growth of NPs in a doped polymer matrix by a thermal annealing Interaction with light Collective

• scillations of

the free e-

Optical response : Localized plasmon resonance

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

Université de Mons

• Tunable absorption in the

visible range

• Optical enhancing

properties

• Antibacterial or anti-

fouling properties

• Non-linear optical

properties

• Random lasers with dyes

(e.g. Rhodamine G)

Interest for Ag-PVA

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• G. V. Ramesh, S. Porel and T. P. Radhakrishnan
• Chem. Soc. Rev., 2009, 38, 2646–2656

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

• C. Guyot | Physics of Materials and Optics Unit

Université de Mons

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• C. Guyot | Physics of Materials and Optics Unit

Spectroscopic Ellipsometry (SE)

𝜍 = 𝑠

𝑞

𝑠

𝑡

= tan Ψ𝑓𝑗Δ = 𝑔(𝑜𝑗, 𝑙𝑗, 𝑈𝑗)

• Non destructive optical

method

• Change of polarization

state after reflexion

• Optical properties (𝑜, 𝑙)

and thicknesses of the layers

• Optical model needed

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

Université de Mons

110°C

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• C. Guyot | Physics of Materials and Optics Unit

Samples

PVA concentration (in water) 2% 8% Dry film thickness ±30 nm ±300 nm Silver to polymer ratio (w:w) 2.5% 25% 2.5% 25% THIN FILMS THICK FILMS

60 min

𝐵𝑕+ + 𝑄𝑊𝐵 → 𝐵𝑕0 − 𝑄𝑊𝐵

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

Université de Mons

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• C. Guyot | Physics of Materials and Optics Unit

± 30 nm

PVA

± 30 nm ± 300 nm

Ag-PVA 25%

𝐓𝐛 = 𝟏. 𝟕𝟕 𝐨𝐧 𝐓𝐛 = 𝟏. 𝟔𝟘 𝐨𝐧 𝐓𝐛 = 𝟏. 𝟒𝟑 𝐨𝐧

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

Université de Mons

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit

Determination of :

• Optical properties (𝑜, 𝑙)
• Thickness of the layers

NPs = homogeneous sphere, radius a and permittivity 𝜗1 in a polymer matrix 𝜗𝑛 𝛽 = 4𝜌𝑏3𝜗𝑛 𝜗1 − 𝜗𝑛 𝜗1 + 2𝜗𝑛 Resonance when 𝜗1 = −2𝜗𝑛 𝑜𝑄𝑊𝐵 = 1.5 𝜗𝑛 = 𝑜𝑄𝑊𝐵 2 = 2.25 ⇒ 𝜗1 = −4.5

𝛽 = cos 2Ψ 𝛾 = sin 2Ψ cos(Δ)

Resonance at 405 nm or 3.1 eV

Spectroscopic ellipsometry

Université de Mons

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit
• 𝐵

Amplitude of the resonance

• Λ0 Position of the resonance peak
• Γ

0 Full-width at half maximum (FWHM)

6 parameters

Spectroscopic ellipsometry

Optical model

Dielectric matrix : Cauchy law (2 param.) LSPR : Lorentzian absorption (3 param.)

Unknown 𝐔, 𝒐(𝝁), 𝒍(𝝁)

Université de Mons

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit

Question : links between the resonance parameters and the experimental parameters ? Approach : Multivariate analysis (4 classes, N > 90)

%Ag (w:w) 2.5% 25% %PVA 2% Thin low (tl) Thin high (tH) 8% Thick low (Tl) Thick high (TH)

Thickness versus A : OK

Université de Mons

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit

? ?

𝚫

𝟏

𝚳𝟏 A Overlapping in the 𝚫

𝟏 − 𝚳𝟏 plane

Different optical behavior between THIN and THICK films ?

Université de Mons

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit

Box-and-whiskers plots (50% of the data in the box)

𝐁 𝚳𝟏 𝚫𝟏

A : strongly correlated to [Ag] L0 : small blue- shift (thin to Thick) G0 : more complicated

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INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit

Support Vector Machines (SVM) or Large Margin Classifiers

Good Good

Best classifier

Margin

Best classifier

Margin

Separable data Overlapping data

OK but … Misclassification = Cost Best classifier = largest margin

Université de Mons

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

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• C. Guyot | Physics of Materials and Optics Unit

Back to the resonance data …

Δ : 2.5% Ag/PVA ratio Ο : 25% Ag/PVA ratio

• Possibility to discriminate the high and low doping level films
• Very efficient way : only 3 samples per plot are misclassified
• Slopes of the optimum classifiers have opposite sign : Thick (−0.36) and

thin films +0.30

0.05 0.10 0.15 0.20 0.25 0.30 0.39 0.40 0.41 0.42 0.43 0.44

(A) Thick films

0 (µm) 0 (µm)

0.00 0.05 0.10 0.15 0.20 0.42 0.43 0.44 0.45 0.46

(B) Thin films

0 (µm) 0 (µm)

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• C. Guyot | Physics of Materials and Optics Unit

Conclusion

• Optical properties of nanocomposites (Ag/PVA) determined from SE data
• Influence of thickness and the Ag concentration on refractive index
• Different behavior of thin and thick films at a given doping level (Ag+)
• Possible explanation : different growth mechanisms resulting in different spatial

distributions of the NPs (2D vs 3D) Silicon Silicon

Thin films Thick films

2D-like matrix

3D-like matrix

INTRODUCTION | MATERIALS & METHODS | RESULTS & DISCUSSION | CONCLUSIONS

Université de Mons

Thank you for your attention

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This work is financially supported by the F.N.R.S. (FRFC project nr 1926111)

• C. Guyot | Physics of Materials and Optics Unit