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

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54 CHAPTER 3. SELF-ORGANIZED NPS ARRAYS: MORPH. ASPECTSafter the deposition of ≈ 250 nm of LiF at different temperatures, within the range between250 ◦ C and 400 ◦ C. In all cases the ripple structure is clearly recognizable and quiteregular. The ripples are very elongated especially at the lowest temperatures (T = 250 ◦ Cfor panel (a) and T = 300 ◦ C for panel (b)), where they extend longer than the imagesize (1μm) and are very straight. This is confirmed by the corresponding 2D Fourierspectra, which rapidly decay in the [001] direction and are much broader in the [1¯10]. Athigher temperatures, T = 350 ◦ C for panel (c) and T = 400 ◦ C for panel (d), the ripplesare more widely spaced but also more irregular; the average length is in-between 500 nmand 1μm, and the grooves appear slightly distorted. Looking at the 2D FFT spectra, thecentral peaks are still mainly spread in the [1¯10] direction, confirming that the uniaxialshape of the ripple remains the dominant feature of the surfaces, however an increasingbroadening in the [001] direction is also observed, indicating a gradual degradation ofthe ripples with the temperature. The broadening of the 2D Fourier spectra in the [1¯10]direction decreases with the ripples periodicity, however this is not an effect of disorder;in fact, the Fourier spectrum intensity for an ideal sawtooth profile with period Λ decaysproportionally to 1/Λ 2 [188].From a statistical analysis of the AFM data, we could extract the average rippleperiodicity Λ along with its standard deviation; we report the results as a function ofT in fig. 3.6. Λ follows an almost linear trend with the temperature, with a rate ofapproximately 1 nm/5 ◦ C: at 250 ◦ C the average ripple spacing is (25±2) nm (fig. 3.5(a));increasingT, Λraisesto(35±3)nmat300 ◦ C(fig.3.5(b)), (45±5)nmat350 ◦ C(fig.3.5(c))and (60±7) nm at 400 ◦ C (fig. 3.5(d)); correspondingly, the standard deviation remainsas low as 3 nm below 300 ◦ C and increases up to ≈ 7 nm at a temperature of 400 ◦ C.60Ripples periodicity [nm]50403020250 300 350 400Substrate temperature [°C]Figure 3.6: Ripples periodicity as a function of the substrate temperature during the LiFhomoepitaxy. Dashed line: linear trend of ≈ 1 nm/5 ◦ C to guide the eye.We did not consider depositions at temperatures outside the 250 ◦ C÷400 ◦ C range.However, lowering the temperature the adatoms mobility decreases and so the faceting ofthe (110) surfaces is harder to occur; on the contrary, at high temperature we are on oneside limited by the sample holder capabilities, on the other side the high adatoms thermalenergy is expected to gradually destroy the ripple coherence.
3.2. 2D ARRAYS OF GOLD NANOPARTICLES 553.2 2D arrays of gold nanoparticlesWe now address the fabrication of bidimensional arrays of metallic nanoparticles (NP),employing the nanostructured LiF substrates as templates to control their size and assisttheir arrangement. The fabrication of the NP arrays follows a two steps procedure. Firsta thin layer of gold is deposited on the rippled LiF templates, and then the NP arrayformation is induced by thermal annealing.As sketched in fig. 3.7(a), the deposition of gold is performed at grazing incidence, inorder to exploit the shadowing effect of the ripples ridges: if the molecular beam of goldreaches the LiF surface at an angle larger than 45 ◦ with respect to the surface normaland perpendicularly to the [001] direction, only one side of the grooves gets exposed tothe beam, and an array of disconnected “wires” is formed. Similarly to the previous case,the deposition is carried out in high vacuum conditions, in order to limit any possiblecontamination of the metallic film. The molecular beam of gold is released from the Mocrucible with a rate of ≈ 0.5 nm/min, and impinges the sample at an angle of incidenceof 60 ◦ . In order to avoid the diffusion of the adatoms across the ripples ridges, thetemperatureiskeptconstantatapproximately100 ◦ Candafterthedepositionthesamplesare immediately cooled down to room temperature. A maximum of ≈ 10 nm of gold hasbeen deposited, as at higher coverages the ripples grooves get completely filled with goldand a continuous layer is formed.[001]60°a.b.[001][001]c.d.400 nmFigure 3.7: Panel a: sketch of the grazing deposition of Au on rippled LiF(110). Panelsb, c, d: AFM images of nanopatterned Lif(110), with different periodicities, after thedeposition of t Au ≈ 5 nm of gold at T ≈ 100 ◦ C; Λ = 45 nm (b, Fourier spectrum in theinset), Λ = 25 nm (c), Λ = 40 nm (d).In fig. 3.7(b) we report an AFM image measured ex-situ on the same sample of
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Università degli studi di GenovaFa
Page 4 and 5: 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
Page 106 and 107:
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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