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

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56 CHAPTER 3. SELF-ORGANIZED NPS ARRAYS: MORPH. ASPECTSfig. 3.5(c) after the deposition of ≈ 5 nm of gold. The surface is still characterizedby coherently aligned elongated structures, indicating that the overgrown layer has followedthe structure of the underlying template. The ordering is also confirmed by the2D Fourier spectrum, where the central peak is still quite stretched in the [0¯11] direction;however, a secondary background is also present, slowly decaying from the central peakin all directions uniformly. This indicates that the metallic stripes are not completelycontinuous but rather composed of close-packed grains; it also suggests that the growth ofgold on LiF does not likely proceed layer-by-layer but probably follows a layer-plus-island(Stranski-Krastanov) mode. Due to the finite size of the AFM tip, it is not possible todeduce from the AFM images whether the islands coalesced or remained separated. Nevertheless,the lateral size of the grains seems to be limited to an upper size of ≈ 15 nm.This is shown in fig. 3.7(c,d), where we report AFM images measured on two sampleswith different ripple spacing and the same amount of deposited gold, ≈ 5 nm; in panel(c), the sample has a periodicity Λ ≈ 25 nm and linear chains of grains are observed,where the grains lateral sizes matches the grooves width; on the contrary, in panels (d)and (b), for a spacing Λ > 40 nm, no island as large as the ripples is found, and insteadmultiple chains are formed inside each groove.a.b.[001] [001]c.[001]d.[001]Figure3.8: AFMimagesofgoldnanoparticlesarraysonnanopatternedLiF(110), followingthe deposition of t Au gold and annealing at T ≈ 100 ◦ C. Panel a: Λ = 30 nm, t Au =4.5 nm. Panel b: Λ = 40 nm, t Au = 4.8 nm. Panel c: Λ = 45 nm, t Au = 5.4 nm. Paneld: Λ = 55 nm, t Au = 7.2 nm.Given the natural tendency of gold to form agglomerates, we can further promotethe dewetting of the “nanowires” (NW) by mild annealing the substrates [189–191]. In
3.2. 2D ARRAYS OF GOLD NANOPARTICLES 57fig. 3.8(c) we report an AFM image measured on the same sample of fig. 3.7(b) after theannealingat400 ◦ C.Comparingthetwoimages, themaineffectoftheannealingprocedurewas a partial melting of the gold grains, which merged together forming larger particles,with a size comparable to the ripples width. The underlying ripple structure assisted theprocess, preventing the spreading of the grains across the ridges, and instead promotingthe formation of parallel chains of nanoparticles, preserving the same periodicity Λ of theripples.Interestingly, the LiF grooves not only guided the particles arrangement, but alsoinfluenced their sizes and shape. This effect can be observed looking at fig. 3.8, wherewe reported AFM images of several NP arrays, annealed at the same temperature of400 ◦ C but fabricated under different growth conditions. Let’s first consider panels (a)and (b). The samples have, respectively, periodicities Λ ≈ 30 nm and ≈ 40 nm, anda similar amount of gold were deposited, ≈ 4.5 nm and ≈ 4.8 nm. In both cases theparticles have an in-plane circular shape, and, although it is not possible to preciselydetermine the particles dimensions due to the convolution of the AFM tip, their typicalsize increases according to the variations of width of the LiF grooves. In panel (c) a samplewith Λ ≈ 45 nm, similar to the sample in panel (b), but t Au ≈ 5.4 nm of deposited goldis shown. In this case, the particles are still confined inside the ripples, and the higheramount of gold led to the formation of more elongated structures, with an average aspectratio of ≈ 1.5. A similar trend is observed increasing further the thickness of gold. Thesample in panel (d) has Λ ≈ 55 nm and t Au ≈ 7.2 nm.The majority of the particles is again as wide as the grooves, however we also observean increased dispersion of particles sizes with respect to the previous cases, both alongand across the ripples. This is probably due to the presence of morphological defects inthe underlying ripple structure, and it happens in concordance with the observations ofthe previous section, where an increasing disorder of the surface morphology was foundat the largest Λ (fig. 3.5(d)). Looking at the shape, the particles now have a maximumaspect ratio of ≈ 2, higher than for sample (c), but many less anisotropic ones are alsovisible; this could be due to the fact that the formation of structures with aspect ratiosgreater than 2 is not favoured, so in these cases a segregation to smaller particles occurs.Then, we can conclude that, for periodicities between 25 nm and 60 nm and thickness ofdeposited gold between 4 nm and 10 nm, the particles width is fixed by the grooves widthwhile the shape (aspect ratio) is determined by the amount of deposited gold.We can interpret this effects by considering the dynamics of the Au nanowires dewetting.If the NW evolve following a Rayleigh-type instability [192], then a linear relationshipbetween the NP spacing (and hence their volume) and the initial NW radiusr (r ∝ (Λ · t Au ) 1/2 ) is expected [193], showing that both the mean spacings along theripples and volume of the NP get larger in correspondence of the increasing Λ and t Au .The simultaneous increase in the aspect ratio can be ascribed to the lateral “constraint”effect exerted by the LiF nanopatterns, that triggers an anisotropic NP growth during thedewetting process.Another unexpected feature of the NP arrays is the relative positions of particlesbelonging to different grooves. As the dewetting induced by the annealing takes placeseparately in each ripple, one may expect no transversal correlation between the positionof the particles in adjacent rows; in contrast, many regions can be distinguished wherethe particles are locally arranged on a rectangular grid. These regions can be emphasizedby looking at the autocorrelations of the AFM images. In fig. 3.9 we report the 2Dautocorrelation plots for the samples of fig. 3.8(b,c); both the figures are characterizedby rectangular patterns of peaks and hollows, with sides oriented in the [1¯10] and [001]
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Università degli studi di GenovaFa
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4 CONTENTS5.2.2 Optical anisotropy
Page 6 and 7: 6 Introductionconfining the EM ener
Page 8 and 9: 8 Introduction
Page 10 and 11: 10 CHAPTER 1. THEORYEλBkFigure 1.1
Page 12 and 13: 12 CHAPTER 1. THEORYfrom which, sep
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
Page 32 and 33: 32 CHAPTER 1. THEORYand then monoto
Page 34 and 35: 34 CHAPTER 1. THEORYposition, has t
Page 36 and 37: 36 CHAPTER 1. THEORYfieldorequivale
Page 38 and 39: 38 CHAPTER 1. THEORY
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
Page 52 and 53: 52 CHAPTER 3. SELF-ORGANIZED NPS AR
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
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106 Acknowledgements
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108 BIBLIOGRAPHY[14] Shouheng Sun,
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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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