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

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102 CHAPTER 6. COMPOSITE MEDIA BASED ON AU/LIF ARRAYS0.250.20T modeL mode0.350.300.25T modeL modeR S0.15R S0.200.150.100.10400 600 800 1000a. [nm]b.400600 [nm]8001000Figure 6.5: Experimental (empty circles) and computed (continuous lines) R S spectra,with plane of incidence parallel (black curves) and perpendicular (red curves) the LiFridges. Left panel: 2D arrays of gold NPs on nanopatterned LiF(110) (Λ = 35 nm,t Au ≈ 5 nm, AFM data in fig. 6.2(a, b)). Right panel: same system after the depositionof a monolayer of Fe 3 O 4 /OA NPs (AFM data in fig. 6.2(c)).monolayer of deposited NPs, the first one was to increase the thickness of the effectivelayer by an amount equal to the mean diameter of the NPs, i.e. ≈ 17 nm. The secondcorrection was to rewrite the dielectric constant ε h of the host including the contributionof the magnetite NPs; we considered the gold NPs embedded in an effective host composedat 50% by LiF (with dielectric constant ε s ) and at 50% by a mixing of air and bulkmagnetite (with dielectric constant ε Fe3O 4, assuming that the dielectric constant of Fe 3 O 4nanoparticles does not differ from the bulk):ε h = 1 2 ε s + 1 2 (f ε Fe 3O 4+(1−f)ε air ) (6.1)where ε air = 1 and f is the magnetite filling factor. The latter is calculated by simplegeometrical considerations: assuming the Fe 3 O 4 /OA nanoparticles arranged on a squarelattice, and in contact with one another, we find one particle per volume of d 3 hydro (d hydrois the hydrodynamic size), while the volume of the magnetite core is 4π/3(d core /2) 3 (d coreis the size of the NPs core); the filling factor f is given by the ratio of the two volumesand, substituting the values obtained from TEM and DLS measurements, results f ≈ 0.3.The calculated R S spectra for the Au NPs array after the Fe 3 O 4 /OA deposition arereported in fig. 6.5(b). Despite the simplifications in treating the magnetite NPs layer,which probably determined the overestimation of the absolute values of reflectivity, thepositions and widths of the LSPs were reproduced in good agreement with the experiments,resulting of λ ∗ L = 645 nm and Γ∗ L = 223 nm for the L mode and λ∗ T = 576 nmand Γ ∗ T = 145 nm for the T mode. In particular, this also supports the consideration thatonly one monolayer of Fe 3 O 4 /OA NPs has been deposited.Inconclusion,wehaveshownthattheself-organizedAu/LiFsystemscouldbefruitfullyexploited as templates for the fabrication of more complex composite structures, that canatleastpartlyretainthemorphologicalcharacteristicsoftheoriginalsystemanditsopticalresponse, adding to these novel functionalities, like magnetic response in this specific case.
ConclusionsIn this thesis, we have discussed the fabrication of self-organized 2-dimensional arrays ofgold nanoparticles, with independently tunable size, shape and periodic arrangement, andthe characterization of their collective plasmonic response.The arrays were fabricated employing self-organized nanopatterned LiF(110) substratesas templates for guiding the particle formation. First, a regular ridge-and-valley(ripple) structure is spontaneously induced by means of homoepitaxial growth of LiF onLiF(110), with a variable periodicity Λ determined by the substrate temperature duringthe deposition. Typical values of Λ = 25÷60 nm by varying the temperature in the rangebetween 250 ◦ C and 450 ◦ C were found.Then, a thin layer of gold was deposited at incidence of θ = 60 ◦ , exploiting theshadow effect of the ripples ridges in order to form disconnected Au “nanowires”. Finally,the samples were mildly annealed at T = 400 ◦ C, promoting the dewetting of the goldnanowires into individual nanoparticles.After the dewetting, the surface of the samples was characterized by parallel chains ofgold nanoparticles, regularly spaced according to the periodicity of the underlying ripples.The particles exhibited lognormal size distribution, with a typical standard deviationlower than 5 nm. Interestingly, we found a partial correlation also between the positionsof particles belonging to adjacent ripples, and rectangular arrangements of Au NPs couldbe observed over large areas of the samples.By tuning the fabrication parameters, we showed the possibility of varying both thesingle NP mean size and in-plane aspect ratio and their mutual spacings in the arrays.The ripples periodicity Λ determined the NPs mean size and spacing in the directiontransversal to the LiF ridges, while the length and spacing along the ripples were relatedto the amount of deposited gold. Performing depositions under different conditions, wecould obtain several 2D arrays of gold NPs with in-plane aspect ratios between 1:1 and1:2, and typical dimensions in the 20÷50 nm range.The 2D arrays of gold NP were optically investigated by means of spectroscopic ellipsometry,reflectivity and transmissivity, in the visible and near-IR frequency range. Theoptical response of the arrays exhibited characteristic absorptions and reflectivity peaks,corresponding to the excitation of localized surface plasmons, i.e. collective oscillations ofthe electron gas confined inside the metallic NPs. Such resonances critically depended onsingle-particles parameters, like shape and size, and on extrinsic factors, like the dielectricenvironment or the inter-particles spacing. We investigated the position and widthof the LSPs for different arrangements of the Au NPs, with particular emphasis on theoptical anisotropy between the longitudinal LSP mode, excited by an electric field alongthe ripples, and the transverse LSP mode, excited perpendicular to the ripples.In order to separately assess the contributions of the intrinsic and collective factorson the plasmonic response of the NP systems, we fabricated and discussed in detail two103
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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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[001][100]50 CHAPTER 3. SELF-ORGANI
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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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