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

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62 CHAPTER 4. SELF-ORGANIZED NPS ARRAYS: OPTICAL PROP.We first consider the LiF(110) samples in the as-received state, whose morphologyhas been reported in fig. 3.2. Macroscopically, they appear completely transparent anduncoloured, without any evident absorption. We report in fig. 4.1 the typical transmission(left panel) and ellipsometric (right panel) spectra for such samples, measured in the280÷1700 nm range. Starting from lower energies, the transmittance (panel (a)) has analmost constant value of 0.95 up to E ≈ 3 eV (corresponding to the wavelength rangeλ = 400÷1700 nm), and then slowly decreases approaching the UV region. No significantspectral features are visible in the 3.5 ÷ 6 eV (200 ÷ 350 nm) range, characteristic ofthe colour centers, confirming the good quality of the crystals. Panel (b) shows Ψ and∆ spectra measured at an angle of incidence θ = 50 ◦ . The values of ∆ are very closeto 180 ◦ , the expected value for an ideally non-absorbing dielectric [126]. Starting fromλ ≈ 800 nm and proceeding to lower wavelengths, ∆ tends to become slightly larger than180 ◦ , probably indicating the presence of a thin surface layer having an optical responseslightly different from the bulk crystal.Since the LiF(110) is a morphologically anisotropic surface (fig. 3.1(d)), we investigatedthe optical behaviour of the LiF substrates varying the orientation of the planeof incidence, with respect to the crystalline axes. Within the experimental accuracy, wefound no significant variations of the ellipsometric response, either setting the plane ofincidence along the [001] or the [1¯10] directions, indicating no influence of the crystalsymmetry on the optical properties of LiF.8.0t LiF= 0 nm184t LiF= 0 nm7.01808.0180t LiF= 60 nm7.0t LiF= 60 nm170 [deg]6.096t LiF= 120 nm [deg]160180t LiF = 120 nm9t LiF= 180 nm160180t LiF= 180 nm61708.0t LiF= 240 nm1806.0170t LiF= 240 nm2.02.53.03.5E [eV]4.04.55.02.02.53.03.5E [eV]4.04.55.0Figure4.2: Ψ(leftpanel)and∆(rightpanel)spectraatincidenceangleθ = 50 ◦ , measuredduring the fabrication of a nanopatterned LiF(110) sample (growth temperature T =300 ◦ C, periodicity Λ = 35 nm), as a function of the deposited thickness t Lif .In analogy with the morphological measurements, we checked the evolution of theLiF optical response, monitoring the ellipsometric spectra during the LiF homoepitaxy
4.2. OPTICAL PROPERTIES OF GOLD NANOPARTICLES ARRAYS 63as a function of the amount of deposited LiF. In fig. 4.2 we show Ψ and ∆ spectra for asample with periodicity Λ ≈ 35 nm (fig. 3.4); like in fig. 3.4, the curves were measuredex-situ at an angle of incidence θ = 50 ◦ , at steps of 60 nm of deposited LiF during thedeposition of a 240 nm thick film. Increasing the thickness of the deposited LiF, we noticethat the average values of the Ψ and ∆ spectra remain almost unchanged, while the mainvariationsconsistintheappearanceofweakoscillationsinthespectra, whichbecomemoreclosely spaced at increasing LiF thickness. These oscillations are due to the interferencebetween the multiple reflections inside the deposited film; in particular, their periodicity isdirectly related to the thickness of the film, making spectroscopic ellipsometry an effectivetechnique to monitor ex-situ the LiF homoepitaxy and to calibrate with a precision of fewnanometers the deposition rate of the effusion cell.8.07.5||184180 [deg]7.0 [deg]1766.5172||6.01.02.03.04.01681.02.03.04.0E [eV]E [eV]Figure 4.3: Ψ (left panel) and ∆ (right panel) spectra, measured at an angle of incidenceθ = 50 ◦ , of a nanopatterned LiF(110) sample, with periodicity Λ = 35 nm, for differentorientations of the sample with respect to the optical plane. Red line: plane of incidenceparallel (||) to the ripples direction. Black line: plane of incidence transverse (⊥) to theLiF ridges.Another characteristic feature induced by the deposition of LiF is a weak but apparentoptical anisotropy between measurements performed with the optical plane parallel to the[001] and [1¯10] directions. This is shown in fig. 4.3, where we report Ψ and ∆ spectrameasured at θ = 50 ◦ with the plane of incidence oriented either parallel (red lines) orperpendicular (black lines) to the ripple direction; the anisotropy is more pronounced for∆, where it is as great as 3 ◦ at the highest energies, while it is barely noticeable for Ψ.Since it is not observed for the bare substrate, we conclude that it must be induced bythe deposited film, and in particular by the intrinsic morphological uniaxial asymmetryof the ripple structure. As is often the case in ellipsometry, the direct interpretation ofthe spectra, and of the corresponding anisotropy, is not a straightforward task, and ananalytical modelling is typically required. We will discuss these effects in the next chapter(§5.1).4.2 Optical properties of gold nanoparticles arraysWe will now address the optical response of the arrays of gold nanostructures. As seenin §1.3, metallic structures with a typical size of few tens of nanometers can sustaincharacteristic plasmonic resonances, whose properties depend on the specific shape andenvironment of the nanoparticles. Here, by tuning the fabrication of the arrays, we explore
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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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Page 14 and 15: 14 CHAPTER 1. THEORY≈ ω 0 like
Page 16 and 17: 16 CHAPTER 1. THEORYε 1 (λ) = N 2
Page 18 and 19: 18 CHAPTER 1. THEORYThe correspondi
Page 20 and 21: 20 CHAPTER 1. THEORYso the induced
Page 22 and 23: 22 CHAPTER 1. THEORYε MG −ε aε
Page 24 and 25: 24 CHAPTER 1. THEORYUV range, this
Page 26 and 27: 26 CHAPTER 1. THEORYwhich is called
Page 28 and 29: 28 CHAPTER 1. THEORYwhere Γ 0 = v
Page 30 and 31: 30 CHAPTER 1. THEORYpoints of the c
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Page 36 and 37: 36 CHAPTER 1. THEORYfieldorequivale
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Page 40 and 41: 40 CHAPTER 2. EXPERIMENTAL METHODSt
Page 44 and 45: 44 CHAPTER 2. EXPERIMENTAL METHODS2
Page 46 and 47: 46 CHAPTER 2. EXPERIMENTAL METHODSs
Page 50 and 51: [001][100]50 CHAPTER 3. SELF-ORGANI
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Page 76 and 77: 76 CHAPTER 5. MODELLING AND ANALYSI
Page 98 and 99: 98 CHAPTER 6. COMPOSITE MEDIA BASED
Page 100 and 101: 100 CHAPTER 6. COMPOSITE MEDIA BASE
Page 102 and 103: 102 CHAPTER 6. COMPOSITE MEDIA BASE
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,
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112 BIBLIOGRAPHY[74] Prashant K. Ja
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114 BIBLIOGRAPHY[103] Tobias Kraus,
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116 BIBLIOGRAPHY[137] F. Brouers, J
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118 BIBLIOGRAPHY[169] Christy L. Ha
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120 BIBLIOGRAPHY[201] G. Baldacchin
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