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

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72 CHAPTER 4. SELF-ORGANIZED NPS ARRAYS: OPTICAL PROP.Λ=30 nm to λ ≈ 620 nm for Λ=40 nm, and the T mode from λ ≈ 550 nm to λ ≈ 585 nm;correspondingly, the intensity of the reflectivities increases, and for Λ=30 nm is nearlytwice as much as for Λ=40 nm.We can qualitatively explain these behaviours by considering the NP particles spacingsand interactions strength. As seen in the previous chapter §3.2, the ripples structure fixesboth the NPs separation in the direction across the ripples, equal to Λ, and the widthof the NPs; instead, the center-to-center distance between adjacent NPs belonging to thesame groove depends on the lengths of the NPs. However, since the particles in fig. 4.11have an in-plane circular section, in these cases the ripples periodicity Λ also affects thelongitudinal separation, so when Λ is reduced the particles grow smaller and get closerin all directions. This has two main consequences: by decreasing the NPs spacings, themutual interactions become stronger, leading to the observed red shift of both the Land T plasmon modes; moreover, the enhancement of the local electric fields promotesstronger induced dipoles, only partially compensated by the reduction of NP volumes,and accordingly a higher reflectivity.LSPR [nm]640600560T modeL mode520303540 [nm]Figure 4.12: Wavelength position of longitudinal (solid red squares) and transverse (openblack squares) LSP resonances for gold nanoparticles on LiF(110) samples with differentripples periodicities Λ and the same amount t Au = 5 nm of deposited gold (see fig. 4.11).The position of the LSP resonances as a function of the periodicity Λ is summarizedin the graph in fig. 4.12. The asymmetry of the L and T modes, despite the in-planespherical shape of the particles, is due to the different NPs spacings in the directionsalong and across the ripples. In fact, the particles are more packed inside the individualgrooves rather than across the ripples, so the EM interactions between the particles arestronger when the electric field is parallel the ripples ridges: this leads to a relative redshift of the L modes with respect to the T modes.Dependence of LSP modes on Au thicknessIn this section we briefly review the effects of the thickness t Au of the deposited gold onthe LSPs, keeping the ripple periodicity Λ fixed. We report in fig. 4.13 sets of reflectivitiesmeasured on different arrays of NPs. The arrays were grown on substrates with a similarperiodicity of Λ ≈ 45 nm, increasing the Au thickness from t Au ≈ 4.5 nm (red curves,sample in fig. 3.8(b)), to t Au ≈ 5.4 nm (blue curves, sample in fig. 3.8(c)), to t Au ≈ 7.2 nm
4.2. OPTICAL PROPERTIES OF AU NANOPARTICLES ARRAYS 73psps0.04R P||t Au= 4.8 nmt Au= 5.4 nmt Au= 7.2 nmL modeER S┴0.30R P0.020.20RS0.004006008001000a. [nm] [nm] b.40060080010000.100.200.15R S||T modeR p┴0.02R S0.10E0.01RP0.0540060080010000.001000c. [nm] [nm] d.Figure 4.13: Sets of reflectivities at θ = 50 ◦ incidence, for s- and p-polarized incident light,measured on nanopatterned LiF(110) samples with similar periodicities Λ ≈ 45 nm, afterthe deposition of different amounts of gold at T ≈ 100 ◦ C and annealing at T = 400 ◦ .Plane of incidence parallel (left panels) and perpendicular (right panels) to the ripples.Excitation of LSP L mode (panels a, b) and T mode (panels c, d).400600800(black curves, sample in fig. 3.8(d)). The corresponding positions of the LSP peaks as afunction of the Au thickness is reported in fig. 4.14.Comparing the spectra, we observe variations of the optical response somewhat similarto the previous case. The intensity of the reflectivities increases with increasing t Au , andcorrespondingly the L mode red-shifts; the T mode, instead, remains almost unchanged.In analogy with the previous case, we can interpret qualitatively these effects. Asdescribed in the previous chapter, the thickness of the deposited gold affects the shapeof the individual NPs; in this case we are dealing with arrays of particles characterizedby different aspect ratios, gradually varying from in-plane spherical to elongated (seefig. 3.8). However, since the arrays were grown on equivalent substrates, the NPs havesimilar widths and separations in the direction across the ripples, the main differencesbeing the lengths and the spacings along the ripples. Recalling the arguments in §1.3.3,we can therefore associate the splitting of the LSP resonances mainly to geometrical
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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
Page 22 and 23: 22 CHAPTER 1. THEORYε MG −ε aε
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Page 26 and 27: 26 CHAPTER 1. THEORYwhich is called
Page 28 and 29: 28 CHAPTER 1. THEORYwhere Γ 0 = v
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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
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Page 98 and 99: 98 CHAPTER 6. COMPOSITE MEDIA BASED
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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,
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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