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

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46 CHAPTER 2. EXPERIMENTAL METHODSspectra is clearly visible a different optical behaviour of the system as a function of thepolarization of light (two split sharp peaks for s- and p-polarized light), while in theellipsometry spectra only one peak is present, broader and shifted in wavelength.2.3 Atomic Force MicroscopyThemorphological characterization ofthesampleshasbeenperformedbymeansofatomicforce microscopy (AFM).Thistechniquewasdevelopedforthefirsttimein1986byBinnig,Quate and Gerber [180], and ever since has become a conventional and versatile methodof surface analysis. AFM is a very high resolution type of scanning probe microscopy,capable of sub-Å vertical resolution and lateral resolution down to atomic resolution. Itis commonly employed for topographic imaging at nanoscale, but more advanced versionsexists which also allow to probe, for example, surface potentials, electric conductance ormagnetic domains.Feedback LoopControllerElectronicsUintermittent-contactregionDetectorElectronicsLaserx,yrepulsive forceattractive forcePosition-sensitivePhotodiodea.zpiezoCantileverSampleb.contactregionnon-contactregiondFigure 2.8: Panel a: sketch of a typical Non-Contact AFM Instrument Configuration.Panel b: schematic representation of the probe-sample interaction potential and AFMoperational modes.A typical AFM setup is depicted in fig. 2.8(a). The microscope consists of a very sharptip, with a few nanometers apex, located near the end of an elastic cantilever. By usingpiezoelectric stages, the tip is brought in close proximity to the sample, until an attractiveor repulsive interaction is established. This force leads to a proportional deflection of thecantilever, which is detected by means of a laser spot reflected from the back of thecantilever onto a position-sensitive photodiode. The relative position between the tipand the sample is controlled by a feedback loop, which drives the piezoelectric scannersaccording to the deflection of the cantilever, and generates the actual AFM images bymaintaining at constant level a particular operational parameter.Depending on the specific situation, several kind of forces can be measured. A typicalprobe-sample interaction potential is shown in fig. 2.8(b), as a function of the relativeseparation. When the tip is far from the sample, more than hundreds of nanometers,attractive long-range interactions prevail, like electrostatic or magnetic dipolar forces.Approaching the surface the attractive forces increase, and at a distance of the order offew nanometers the major contribution is from intermolecular Van der Waals interactions.
2.3. ATOMIC FORCE MICROSCOPY 47Reducing further the distance, the tip starts to penetrate the sample and locally deformsthe surface; elastic repulsion forces are now dominating, therefore the potential reaches aminimum, where the attractive and repulsive forces are equally balanced, and then rapidlydiverges.SurfaceCantileverSlow scan axisFast scan axisFigure 2.9: Sketch of an AFM acquisition. The sample is scanned line-by-line in the fastaxis direction and the tip-sample interaction is measured at a discrete set of points (blackcircles). At the end of each scan, the tip is shifted along the slow axis and brought backat the start of the line.AFM acquisitions are performed by running the tip over a specified area of the sample,and measuring the interaction with the surface at a discrete set of points, as sketched infig. 2.9. Depending on the kind of measurement and on the nature of the sample, severaloperational modes are available. In order to acquire topographic images of surfaces themost common ones are contact, non-contact and tapping modes.In contact AFM [180], the tip works in the repulsive range of the interaction potentialand is dragged across the surface. During the scanning, the deflection of the cantilever iskept constant, and the corrections applied to the vertical piezoelectric stage provide thetopographic information. This technique is usually fast and very accurate, but for soft orvery rough samples the strong repulsive forces (of the order of nN) can lead to irreversibledegradations of both the tip and the surface.Non-contact mode [181] resolves this issue by increasing the tip-sample separation toabout 50÷150 Å, in the range of the attractive Van der Waals forces. These forces areconsiderably weaker than the ones established in contact mode, so the tip is given a smalloscillation, and AC detection methods are used to detect the small changes in amplitude,phase, or frequency of the oscillating cantilever, in response to force gradients from thesample. This mode has the advantage that the tip is never in contact with the sampleand therefore can not sketch or destroy it, however it provides lower resolution and inpresence of a fluid contaminant layer the oscillating probe can be trapped in the fluid,failing to image the true surface of the sample.Tapping AFM is an high resolution technique which overcomes both the problemsassociated with the degradation of the sample and with friction and adhesion. Like noncontactAFM, the tip is not static, however the oscillation amplitude is much higher(typically in the range 20÷200 nm), the tip being alternately in contact with the surface,to provide high resolution, and lifted off, to avoid the alteration of the surface. As thetip approaches the sample, the force gradient alters the cantilever motion, modifying theamplitude, the resonance frequency, and the phase of the oscillations. During scanning,the feedback system adjusts the relative distance between the tip and the sample in orderto maintain one of these parameters at a constant level, and correspondingly generatesthe AFM image. Usually the topographic surface features are detected by monitoring
Page 1: 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
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Page 52 and 53: 52 CHAPTER 3. SELF-ORGANIZED NPS AR
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96 CHAPTER 5. MODELLING AND ANALYSI
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98 CHAPTER 6. COMPOSITE MEDIA BASED
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100 CHAPTER 6. COMPOSITE MEDIA BASE
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102 CHAPTER 6. COMPOSITE MEDIA BASE
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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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