Morphology and plasmonic properties of self-organized arrays of ...

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100 CHAPTER 6. COMPOSITE MEDIA BASED ON AU/LIF ARRAYSThepatternisparticularlyemphasizedbythecorrespondingFourierspectraldensity(insetof fig. 6.2(d)), which is characterized by two sharp peaks confined along the [1¯10] and the[001] directions, indicating the presence of periodic features both along and across theripples. These observations suggest that, even if the Fe 3 O 4 /OA NPs do not follow the AuNPs arrangement with a one-to-one correspondence, still the Au NPs provide preferentialsites for the adsorption of the magnetic NPs, so that their arrangement can be effectivelyreproduced by groups of few Fe 3 O 4 /OA NPs.Similarly to the previous cases, the optical response of the sample was optically characterizedby means of s- and p-polarized light reflectivity, with the plane of incidenceeither parallel or perpendicular to the LiF ripples. The R S and R P spectra acquired atan incidence angle of θ = 50 ◦ , before and after the Fe 3 O 4 /OA deposition, are reported infig. 6.3.The presence of the magnetite layer affects the LSPs by systematically redshiftingthe resonances and increasing their width; the overall intensity of the reflectivity is alsopspsbefore depositionafter deposition0.04R P||0.30R S┴R P0.02R S0.200.100.004006008001000a. [nm]b.400600 800 [nm]10000.24R S||0.05R p┴R S0.16R P0.030.080.01400600 800 [nm]1000c. d.400600 800 [nm]1000Figure 6.3: Reflectivities, at θ = 50 ◦ , measured before (red curves) and after (blackcurves) the deposition of ≈ 1 monolayer of Fe 3 O 4 OA NPs on 2D arrays of gold NPs (seefig. 6.2). Plane of incidence either parallel (left panels) and perpendicular (right panels) tothe ripples. EM excitation along (panels (a), (b)) and across (panels (c), (d)) the ripples.
101increased. Before the deposition (red curves in fig. 6.3), the longitudinal LSP mode isfound at λ L = 620 nm, with a width of Γ L = 170 nm; the transverse mode is insteadlocated at λ T = 570 nm and has a width of Γ T = 125 nm. Following the deposition (blackcurves in fig. 6.3), both modes redshift by few tens of nm, the L mode at λ ∗ L = 645 nmand the T mode at λ ∗ T = 585 nm, while the corresponding widths become Γ∗ L = 215 nmand Γ ∗ T = 145 nm, respectively.This trend is in accordance with the qualitative considerations in §1.3.3, where wediscussed the dependence of the dielectric environment on the LSP resonances. In particular,the simultaneous redshift of the resonances can be explained in terms of the Fröhlichcondition Re [ ε L,Tm (λ L,T ) ] = −2ε h (see eq. (1.45)), where ε m , ε L,Thand λ L,T are thedielectric constant of the individual Au NPs and of the host and the position of the resonances,respectively. ε m is shown in fig. 5.6(b), and its real part has a negative slopefor increasing wavelengths; in fig. 6.4 we report instead the dielectric constant of bulkmagnetite, extracted from ref. [220]. In analogy with the model of the previous chapter,in first approximation the dielectric constant ε h can be expressed as a linear combinationof the dielectric constants of the materials surrounding the Au NPs; therefore, followingthe deposition of the Fe 3 O 4 /OA NPs, ε h is expected to increase, because the real partof the dielectric constant of magnetite reads about ε 1 = 5 in correspondence of the goldnanoparticles LSP resonances. Then, according to the Fröhlich condition, if ε h increasesand ε m has a negative slope as a function of the wavelength, the LSP resonances of thegold NPs must red shift when embedded in the Fe 3 O 4 /OA layer.64.053.5Re []43.0Im []32.522.040060080010001200 [nm]Figure 6.4: Real (red line) and imaginary (black line) parts of the dielectric constant ofbulk magnetite in the visible and near-IR range, from ref. [220].In order to quantitatively evaluate the effects of the Fe 3 O 4 /OA NPs on the plasmonicresponse of the Au NPs array, we can apply the theoretical framework developed in theprevious chapter to the current system. In fig. 6.5(a) we report the computed R S spectrafor the “bare” Au/LiF nanostructures, compared to the corresponding experimentalvalues, for the L and T LSP modes, at an angle of incidence of θ = 50 ◦ . In analogywith before, the morphological parameters employed for the calculations were obtainedfrom the analysis of the AFM data: for the NPs spacings across and along the ripples weused d x = d y = 33 nm, while the values of the ellipsoids semiaxes were a x = 11.2 nm,a y = 14.0 nm and a z = 6.4 nm.The corresponding spectra in presence of the Fe 3 O 4 /OA NPs were computed keepingthe same input parameters, except two specific modifications. Considering a single
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Università degli studi di GenovaFa
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4 CONTENTS5.2.2 Optical anisotropy
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6 Introductionconfining the EM ener
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8 Introduction
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10 CHAPTER 1. THEORYEλBkFigure 1.1
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12 CHAPTER 1. THEORYfrom which, sep
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14 CHAPTER 1. THEORY≈ ω 0 like
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16 CHAPTER 1. THEORYε 1 (λ) = N 2
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18 CHAPTER 1. THEORYThe correspondi
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20 CHAPTER 1. THEORYso the induced
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22 CHAPTER 1. THEORYε MG −ε aε
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24 CHAPTER 1. THEORYUV range, this
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26 CHAPTER 1. THEORYwhich is called
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28 CHAPTER 1. THEORYwhere Γ 0 = v
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30 CHAPTER 1. THEORYpoints of the c
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32 CHAPTER 1. THEORYand then monoto
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34 CHAPTER 1. THEORYposition, has t
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36 CHAPTER 1. THEORYfieldorequivale
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38 CHAPTER 1. THEORY
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40 CHAPTER 2. EXPERIMENTAL METHODSt
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44 CHAPTER 2. EXPERIMENTAL METHODS2
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46 CHAPTER 2. EXPERIMENTAL METHODSs
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Page 104 and 105: 104 Conclusionsparticular cases of
Page 106 and 107: 106 Acknowledgements
Page 108 and 109: 108 BIBLIOGRAPHY[14] Shouheng Sun,
Page 110 and 111: 110 BIBLIOGRAPHY[44] Q A Pankhurst,
Page 112 and 113: 112 BIBLIOGRAPHY[74] Prashant K. Ja
Page 114 and 115: 114 BIBLIOGRAPHY[103] Tobias Kraus,
Page 116 and 117: 116 BIBLIOGRAPHY[137] F. Brouers, J
Page 118 and 119: 118 BIBLIOGRAPHY[169] Christy L. Ha
Page 120 and 121: 120 BIBLIOGRAPHY[201] G. Baldacchin
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